different types of scientific research papers

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Types of research papers

Analytical research paper

Argumentative or persuasive paper, definition paper, compare and contrast paper, cause and effect paper, interpretative paper, experimental research paper, survey research paper, frequently asked questions about the different types of research papers, related articles.

There are multiple different types of research papers. It is important to know which type of research paper is required for your assignment, as each type of research paper requires different preparation. Below is a list of the most common types of research papers.

➡️ Read more:  What is a research paper?

In an analytical research paper you:

• pose a question
• collect relevant data from other researchers
• analyze their different viewpoints

You focus on the findings and conclusions of other researchers and then make a personal conclusion about the topic. It is important to stay neutral and not show your own negative or positive position on the matter.

The argumentative paper presents two sides of a controversial issue in one paper. It is aimed at getting the reader on the side of your point of view.

You should include and cite findings and arguments of different researchers on both sides of the issue, but then favor one side over the other and try to persuade the reader of your side. Your arguments should not be too emotional though, they still need to be supported with logical facts and statistical data.

Tip: Avoid expressing too much emotion in a persuasive paper.

The definition paper solely describes facts or objective arguments without using any personal emotion or opinion of the author. Its only purpose is to provide information. You should include facts from a variety of sources, but leave those facts unanalyzed.

Compare and contrast papers are used to analyze the difference between two:

Make sure to sufficiently describe both sides in the paper, and then move on to comparing and contrasting both thesis and supporting one.

Cause and effect papers are usually the first types of research papers that high school and college students write. They trace probable or expected results from a specific action and answer the main questions "Why?" and "What?", which reflect effects and causes.

In business and education fields, cause and effect papers will help trace a range of results that could arise from a particular action or situation.

An interpretative paper requires you to use knowledge that you have gained from a particular case study, for example a legal situation in law studies. You need to write the paper based on an established theoretical framework and use valid supporting data to back up your statement and conclusion.

This type of research paper basically describes a particular experiment in detail. It is common in fields like:

Experiments are aimed to explain a certain outcome or phenomenon with certain actions. You need to describe your experiment with supporting data and then analyze it sufficiently.

This research paper demands the conduction of a survey that includes asking questions to respondents. The conductor of the survey then collects all the information from the survey and analyzes it to present it in the research paper.

➡️ Ready to start your research paper? Take a look at our guide on how to start a research paper .

In an analytical research paper, you pose a question and then collect relevant data from other researchers to analyze their different viewpoints. You focus on the findings and conclusions of other researchers and then make a personal conclusion about the topic.

The definition paper solely describes facts or objective arguments without using any personal emotion or opinion of the author. Its only purpose is to provide information.

Cause and effect papers are usually the first types of research papers that high school and college students are confronted with. The answer questions like "Why?" and "What?", which reflect effects and causes. In business and education fields, cause and effect papers will help trace a range of results that could arise from a particular action or situation.

This type of research paper describes a particular experiment in detail. It is common in fields like biology, chemistry or physics. Experiments are aimed to explain a certain outcome or phenomenon with certain actions.

Basic structure and types of scientific papers

Affiliation.

• 1 Singapore Medical Journal, Singapore. [email protected]
• PMID: 18695858

The basic structure of a scientific paper is summarised by the acronym IMRAD. Many types of papers are published in medical journals. These include original articles, case reports, technical notes, pictorial essays, reviews, commentaries and editorials. Authors should be aware that each type of paper is specific in nature, serve a distinct purpose, and is judged by different criteria. Authors submitting their work in the most appropriate format will be able to maximise their material and enhance the chances of manuscript acceptance.

• Biomedical Research
• Journalism, Medical / standards
• Periodicals as Topic / standards
• Professional Competence
• Publishing / standards*

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Scientific Papers

Scientific papers are for sharing your own original research work with other scientists or for reviewing the research conducted by others. As such, they are critical to the evolution of modern science, in which the work of one scientist builds upon that of others. To reach their goal, papers must aim to inform, not impress. They must be highly readable — that is, clear, accurate, and concise. They are more likely to be cited by other scientists if they are helpful rather than cryptic or self-centered.

Scientific papers typically have two audiences: first, the referees, who help the journal editor decide whether a paper is suitable for publication; and second, the journal readers themselves, who may be more or less knowledgeable about the topic addressed in the paper. To be accepted by referees and cited by readers, papers must do more than simply present a chronological account of the research work. Rather, they must convince their audience that the research presented is important, valid, and relevant to other scientists in the same field. To this end, they must emphasize both the motivation for the work and the outcome of it, and they must include just enough evidence to establish the validity of this outcome.

Papers that report experimental work are often structured chronologically in five sections: first, Introduction ; then Materials and Methods , Results , and Discussion (together, these three sections make up the paper's body); and finally, Conclusion .

• The Introduction section clarifies the motivation for the work presented and prepares readers for the structure of the paper.
• The Materials and Methods section provides sufficient detail for other scientists to reproduce the experiments presented in the paper. In some journals, this information is placed in an appendix, because it is not what most readers want to know first.
• The Results and Discussion sections present and discuss the research results, respectively. They are often usefully combined into one section, however, because readers can seldom make sense of results alone without accompanying interpretation — they need to be told what the results mean.
• The Conclusion section presents the outcome of the work by interpreting the findings at a higher level of abstraction than the Discussion and by relating these findings to the motivation stated in the Introduction .

(Papers reporting something other than experiments, such as a new method or technology, typically have different sections in their body, but they include the same Introduction and Conclusion sections as described above.)

Although the above structure reflects the progression of most research projects, effective papers typically break the chronology in at least three ways to present their content in the order in which the audience will most likely want to read it. First and foremost, they summarize the motivation for, and the outcome of, the work in an abstract, located before the Introduction . In a sense, they reveal the beginning and end of the story — briefly — before providing the full story. Second, they move the more detailed, less important parts of the body to the end of the paper in one or more appendices so that these parts do not stand in the readers' way. Finally, they structure the content in the body in theorem-proof fashion, stating first what readers must remember (for example, as the first sentence of a paragraph) and then presenting evidence to support this statement.

The introduction

• First, provide some context to orient those readers who are less familiar with your topic and to establish the importance of your work.
• Second, state the need for your work, as an opposition between what the scientific community currently has and what it wants.
• Third, indicate what you have done in an effort to address the need (this is the task).
• Finally, preview the remainder of the paper to mentally prepare readers for its structure, in the object of the document.

Context and need

At the beginning of the Introduction section, the context and need work together as a funnel: They start broad and progressively narrow down to the issue addressed in the paper. To spark interest among your audience — referees and journal readers alike — provide a compelling motivation for the work presented in your paper: The fact that a phenomenon has never been studied before is not, in and of itself, a reason to study that phenomenon.

Write the context in a way that appeals to a broad range of readers and leads into the need. Do not include context for the sake of including context: Rather, provide only what will help readers better understand the need and, especially, its importance. Consider anchoring the context in time, using phrases such as recently , in the past 10 years , or since the early 1990s . You may also want to anchor your context in space (either geographically or within a given research field).

Convey the need for the work as an opposition between actual and desired situations. Start by stating the actual situation (what we have) as a direct continuation of the context. If you feel you must explain recent achievements in much detail — say, in more than one or two paragraphs — consider moving the details to a section titled State of the art (or something similar) after the Introduction , but do provide a brief idea of the actual situation in the Introduction . Next, state the desired situation (what we want). Emphasize the contrast between the actual and desired situations with such words as but , however, or unfortunately .

One elegant way to express the desired part of the need is to combine it with the task in a single sentence. This sentence expresses first the objective, then the action undertaken to reach this objective, thus creating a strong and elegant connection between need and task. Here are three examples of such a combination:

To confirm this assumption , we studied the effects of a range of inhibitors of connexin channels . . . on . . .

To assess whether such multiple-coil sensors perform better than single-signal ones , we tested two of them — the DuoPXK and the GEMM3 — in a field where . . . To form a better view of the global distribution and infectiousness of this pathogen , we examined 1645 postmetamorphic and adult amphibians collected from 27 countries between 1984 and 2006 for the presence of . . .

Task and object

An Introduction is usually clearer and more logical when it separates what the authors have done (the task) from what the paper itself attempts or covers (the object of the document). In other words, the task clarifies your contribution as a scientist, whereas the object of the document prepares readers for the structure of the paper, thus allowing focused or selective reading.

For the task,

• use whoever did the work (normally, you and your colleagues) as the subject of the sentence: we or perhaps the authors;
• use a verb expressing a research action: measured , calculated , etc.;
• set that verb in the past tense.

The three examples below are well-formed tasks.

To confirm this assumption, we studied the effects of a range of inhibitors of connexin channels, such as the connexin mimetic peptides Gap26 and Gap27 and anti-peptide antibodies, on calcium signaling in cardiac cells and HeLa cells expressing connexins.

During controlled experiments, we investigated the influence of the HMP boundary conditions on liver flows.

To tackle this problem, we developed a new software verification technique called oblivious hashing, which calculates the hash values based on the actual execution of the program.

The list below provides examples of verbs that express research actions:

For the object of the document,

• use the document itself as the subject of the sentence: this paper , this letter , etc.;
• use a verb expressing a communication action: presents , summarizes , etc.;
• set the verb in the present tense.

The three examples below are suitable objects of the document for the three tasks shown above, respectively.

This paper clarifies the role of CxHc on calcium oscillations in neonatal cardiac myocytes and calcium transients induced by ATP in HL-cells originated from cardiac atrium and in HeLa cells expressing connexin 43 or 26. This paper presents the flow effects induced by increasing the hepatic-artery pressure and by obstructing the vena cava inferior. This paper discusses the theory behind oblivious hashing and shows how this approach can be applied for local software tamper resistance and remote code authentication.

The list below provides examples of verbs that express communication actions:

Even the most logical structure is of little use if readers do not see and understand it as they progress through a paper. Thus, as you organize the body of your paper into sections and perhaps subsections, remember to prepare your readers for the structure ahead at all levels. You already do so for the overall structure of the body (the sections) in the object of the document at the end of the Introduction . You can similarly prepare your readers for an upcoming division into subsections by introducing a global paragraph between the heading of a section and the heading of its first subsection. This paragraph can contain any information relating to the section as a whole rather than particular subsections, but it should at least announce the subsections, whether explicitly or implicitly. An explicit preview would be phrased much like the object of the document: "This section first . . . , then . . . , and finally . . . "

Although papers can be organized into sections in many ways, those reporting experimental work typically include Materials and Methods , Results , and Discussion in their body. In any case, the paragraphs in these sections should begin with a topic sentence to prepare readers for their contents, allow selective reading, and — ideally — get a message across.

Materials and methods

Results and discussion.

When reporting and discussing your results, do not force your readers to go through everything you went through in chronological order. Instead, state the message of each paragraph upfront: Convey in the first sentence what you want readers to remember from the paragraph as a whole. Focus on what happened, not on the fact that you observed it. Then develop your message in the remainder of the paragraph, including only that information you think you need to convince your audience.

The conclusion

At the end of your Conclusion , consider including perspectives — that is, an idea of what could or should still be done in relation to the issue addressed in the paper. If you include perspectives, clarify whether you are referring to firm plans for yourself and your colleagues ("In the coming months, we will . . . ") or to an invitation to readers ("One remaining question is . . . ").

If your paper includes a well-structured Introduction and an effective abstract, you need not repeat any of the Introduction in the Conclusion . In particular, do not restate what you have done or what the paper does. Instead, focus on what you have found and, especially, on what your findings mean. Do not be afraid to write a short Conclusion section: If you can conclude in just a few sentences given the rich discussion in the body of the paper, then do so. (In other words, resist the temptation to repeat material from the Introduction just to make the Conclusio n longer under the false belief that a longer Conclusion will seem more impressive.)

The abstract

Typically, readers are primarily interested in the information presented in a paper's Introduction and Conclusion sections. Primarily, they want to know the motivation for the work presented and the outcome of this work. Then (and only then) the most specialized among them might want to know the details of the work. Thus, an effective abstract focuses on motivation and outcome; in doing so, it parallels the paper's Introduction and Conclusion .

Accordingly, you can think of an abstract as having two distinct parts — motivation and outcome — even if it is typeset as a single paragraph. For the first part, follow the same structure as the Introduction section of the paper: State the context, the need, the task, and the object of the document. For the second part, mention your findings (the what ) and, especially, your conclusion (the so what — that is, the interpretation of your findings); if appropriate, end with perspectives, as in the Conclusion section of your paper.

Although the structure of the abstract parallels the Introduction and Conclusion sections, it differs from these sections in the audience it addresses. The abstract is read by many different readers, from the most specialized to the least specialized among the target audience. In a sense, it should be the least specialized part of the paper. Any scientist reading it should be able to understand why the work was carried out and why it is important (context and need), what the authors did (task) and what the paper reports about this work (object of the document), what the authors found (findings), what these findings mean (the conclusion), and possibly what the next steps are (perspectives). In contrast, the full paper is typically read by specialists only; its Introduction and Conclusion are more detailed (that is, longer and more specialized) than the abstract.

An effective abstract stands on its own — it can be understood fully even when made available without the full paper. To this end, avoid referring to figures or the bibliography in the abstract. Also, introduce any acronyms the first time you use them in the abstract (if needed), and do so again in the full paper (see Mechanics: Using abbreviations ).

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Types of Research – Explained with Examples

• By DiscoverPhDs
• October 2, 2020

Types of Research

Research is about using established methods to investigate a problem or question in detail with the aim of generating new knowledge about it.

It is a vital tool for scientific advancement because it allows researchers to prove or refute hypotheses based on clearly defined parameters, environments and assumptions. Due to this, it enables us to confidently contribute to knowledge as it allows research to be verified and replicated.

Knowing the types of research and what each of them focuses on will allow you to better plan your project, utilises the most appropriate methodologies and techniques and better communicate your findings to other researchers and supervisors.

Classification of Types of Research

There are various types of research that are classified according to their objective, depth of study, analysed data, time required to study the phenomenon and other factors. It’s important to note that a research project will not be limited to one type of research, but will likely use several.

According to its Purpose

Theoretical research.

Theoretical research, also referred to as pure or basic research, focuses on generating knowledge , regardless of its practical application. Here, data collection is used to generate new general concepts for a better understanding of a particular field or to answer a theoretical research question.

Results of this kind are usually oriented towards the formulation of theories and are usually based on documentary analysis, the development of mathematical formulas and the reflection of high-level researchers.

Applied Research

Here, the goal is to find strategies that can be used to address a specific research problem. Applied research draws on theory to generate practical scientific knowledge, and its use is very common in STEM fields such as engineering, computer science and medicine.

This type of research is subdivided into two types:

• Technological applied research : looks towards improving efficiency in a particular productive sector through the improvement of processes or machinery related to said productive processes.
• Scientific applied research : has predictive purposes. Through this type of research design, we can measure certain variables to predict behaviours useful to the goods and services sector, such as consumption patterns and viability of commercial projects.

According to your Depth of Scope

Exploratory research.

Exploratory research is used for the preliminary investigation of a subject that is not yet well understood or sufficiently researched. It serves to establish a frame of reference and a hypothesis from which an in-depth study can be developed that will enable conclusive results to be generated.

Because exploratory research is based on the study of little-studied phenomena, it relies less on theory and more on the collection of data to identify patterns that explain these phenomena.

Descriptive Research

The primary objective of descriptive research is to define the characteristics of a particular phenomenon without necessarily investigating the causes that produce it.

In this type of research, the researcher must take particular care not to intervene in the observed object or phenomenon, as its behaviour may change if an external factor is involved.

Explanatory Research

Explanatory research is the most common type of research method and is responsible for establishing cause-and-effect relationships that allow generalisations to be extended to similar realities. It is closely related to descriptive research, although it provides additional information about the observed object and its interactions with the environment.

Correlational Research

The purpose of this type of scientific research is to identify the relationship between two or more variables. A correlational study aims to determine whether a variable changes, how much the other elements of the observed system change.

According to the Type of Data Used

Qualitative research.

Qualitative methods are often used in the social sciences to collect, compare and interpret information, has a linguistic-semiotic basis and is used in techniques such as discourse analysis, interviews, surveys, records and participant observations.

In order to use statistical methods to validate their results, the observations collected must be evaluated numerically. Qualitative research, however, tends to be subjective, since not all data can be fully controlled. Therefore, this type of research design is better suited to extracting meaning from an event or phenomenon (the ‘why’) than its cause (the ‘how’).

Quantitative Research

Quantitative research study delves into a phenomena through quantitative data collection and using mathematical, statistical and computer-aided tools to measure them . This allows generalised conclusions to be projected over time.

According to the Degree of Manipulation of Variables

Experimental research.

It is about designing or replicating a phenomenon whose variables are manipulated under strictly controlled conditions in order to identify or discover its effect on another independent variable or object. The phenomenon to be studied is measured through study and control groups, and according to the guidelines of the scientific method.

Non-Experimental Research

Also known as an observational study, it focuses on the analysis of a phenomenon in its natural context. As such, the researcher does not intervene directly, but limits their involvement to measuring the variables required for the study. Due to its observational nature, it is often used in descriptive research.

Quasi-Experimental Research

It controls only some variables of the phenomenon under investigation and is therefore not entirely experimental. In this case, the study and the focus group cannot be randomly selected, but are chosen from existing groups or populations . This is to ensure the collected data is relevant and that the knowledge, perspectives and opinions of the population can be incorporated into the study.

According to the Type of Inference

Deductive investigation.

In this type of research, reality is explained by general laws that point to certain conclusions; conclusions are expected to be part of the premise of the research problem and considered correct if the premise is valid and the inductive method is applied correctly.

Inductive Research

In this type of research, knowledge is generated from an observation to achieve a generalisation. It is based on the collection of specific data to develop new theories.

Hypothetical-Deductive Investigation

It is based on observing reality to make a hypothesis, then use deduction to obtain a conclusion and finally verify or reject it through experience.

According to the Time in Which it is Carried Out

Longitudinal study (also referred to as diachronic research).

It is the monitoring of the same event, individual or group over a defined period of time. It aims to track changes in a number of variables and see how they evolve over time. It is often used in medical, psychological and social areas .

Cross-Sectional Study (also referred to as Synchronous Research)

Cross-sectional research design is used to observe phenomena, an individual or a group of research subjects at a given time.

According to The Sources of Information

Primary research.

This fundamental research type is defined by the fact that the data is collected directly from the source, that is, it consists of primary, first-hand information.

Secondary research

Unlike primary research, secondary research is developed with information from secondary sources, which are generally based on scientific literature and other documents compiled by another researcher.

According to How the Data is Obtained

Documentary (cabinet).

Documentary research, or secondary sources, is based on a systematic review of existing sources of information on a particular subject. This type of scientific research is commonly used when undertaking literature reviews or producing a case study.

Field research study involves the direct collection of information at the location where the observed phenomenon occurs.

From Laboratory

Laboratory research is carried out in a controlled environment in order to isolate a dependent variable and establish its relationship with other variables through scientific methods.

Mixed-Method: Documentary, Field and/or Laboratory

Mixed research methodologies combine results from both secondary (documentary) sources and primary sources through field or laboratory research.

The purpose of research is to enhance society by advancing knowledge through developing scientific theories, concepts and ideas – find out more on what this involves.

The answer is simple: there is no age limit for doing a PhD; in fact, the oldest known person to have gained a PhD in the UK was 95 years old.

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Kyle’s in the first year of his PhD at the University of Texas in Austin. His research interests are in the development of algorithms for advanced mobility for legged robotics and computer vision for exploration of extreme environments.

Rebecca recently finished her PhD at the University of York. Her research investigated the adaptations that occur in the symbiosis between the tsetse fly and its bacterial microbiome.

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Types of journal articles

It is helpful to familiarise yourself with the different types of articles published by journals. Although it may appear there are a large number of types of articles published due to the wide variety of names they are published under, most articles published are one of the following types; Original Research, Review Articles, Short reports or Letters, Case Studies, Methodologies.

Original Research:

This is the most common type of journal manuscript used to publish full reports of data from research. It may be called an  Original Article, Research Article, Research, or just  Article, depending on the journal. The Original Research format is suitable for many different fields and different types of studies. It includes full Introduction, Methods, Results, and Discussion sections.

Short reports or Letters:

These papers communicate brief reports of data from original research that editors believe will be interesting to many researchers, and that will likely stimulate further research in the field. As they are relatively short the format is useful for scientists with results that are time sensitive (for example, those in highly competitive or quickly-changing disciplines). This format often has strict length limits, so some experimental details may not be published until the authors write a full Original Research manuscript. These papers are also sometimes called Brief communications .

Review Articles:

Review Articles provide a comprehensive summary of research on a certain topic, and a perspective on the state of the field and where it is heading. They are often written by leaders in a particular discipline after invitation from the editors of a journal. Reviews are often widely read (for example, by researchers looking for a full introduction to a field) and highly cited. Reviews commonly cite approximately 100 primary research articles.

TIP: If you would like to write a Review but have not been invited by a journal, be sure to check the journal website as some journals to not consider unsolicited Reviews. If the website does not mention whether Reviews are commissioned it is wise to send a pre-submission enquiry letter to the journal editor to propose your Review manuscript before you spend time writing it.  

Case Studies:

These articles report specific instances of interesting phenomena. A goal of Case Studies is to make other researchers aware of the possibility that a specific phenomenon might occur. This type of study is often used in medicine to report the occurrence of previously unknown or emerging pathologies.

Methodologies or Methods

These articles present a new experimental method, test or procedure. The method described may either be completely new, or may offer a better version of an existing method. The article should describe a demonstrable advance on what is currently available.

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Scientific Research and Communication — Types of Scientific Papers

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What types of articles are published in scientific journals?

Journal articles in the sciences are almost always a write-up of grant-funded laboratory or field research. Each article provides a brief overview of the research study, a description of methods used, results, and a conclusion. A small portion of science articles are 'review' articles; these are articles that summarize research studies. Journal articles are peer-reviewed. A growing number of so-called "pre-prints" are beginning to be published in science fields.

Source of slide above: Research4Life.org 

Link to Slideshow, "How to Read A Scientific Research Paper" from Reseach4Life

• Anatomy of an Article
• How to Seriously Read a Scientific Paper A useful article from Science magazine on how to read a scientific journal article.
• How to Read a Scientific Paper Powerpoint from Research 4 Life about reading scientific journal articles
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Organizing Your Social Sciences Research Paper

• Types of Research Designs
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Choosing a Title
• Making an Outline
• Paragraph Development
• Research Process Video Series
• Executive Summary
• The C.A.R.S. Model
• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
• Content Alert Services
• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
• Quantitative Methods
• Insiderness
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• Common Grammar Mistakes
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• Further Readings
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• Bibliography

Introduction

Before beginning your paper, you need to decide how you plan to design the study .

The research design refers to the overall strategy and analytical approach that you have chosen in order to integrate, in a coherent and logical way, the different components of the study, thus ensuring that the research problem will be thoroughly investigated. It constitutes the blueprint for the collection, measurement, and interpretation of information and data. Note that the research problem determines the type of design you choose, not the other way around!

De Vaus, D. A. Research Design in Social Research . London: SAGE, 2001; Trochim, William M.K. Research Methods Knowledge Base. 2006.

General Structure and Writing Style

The function of a research design is to ensure that the evidence obtained enables you to effectively address the research problem logically and as unambiguously as possible . In social sciences research, obtaining information relevant to the research problem generally entails specifying the type of evidence needed to test the underlying assumptions of a theory, to evaluate a program, or to accurately describe and assess meaning related to an observable phenomenon.

With this in mind, a common mistake made by researchers is that they begin their investigations before they have thought critically about what information is required to address the research problem. Without attending to these design issues beforehand, the overall research problem will not be adequately addressed and any conclusions drawn will run the risk of being weak and unconvincing. As a consequence, the overall validity of the study will be undermined.

The length and complexity of describing the research design in your paper can vary considerably, but any well-developed description will achieve the following :

• Identify the research problem clearly and justify its selection, particularly in relation to any valid alternative designs that could have been used,
• Review and synthesize previously published literature associated with the research problem,
• Clearly and explicitly specify hypotheses [i.e., research questions] central to the problem,
• Effectively describe the information and/or data which will be necessary for an adequate testing of the hypotheses and explain how such information and/or data will be obtained, and
• Describe the methods of analysis to be applied to the data in determining whether or not the hypotheses are true or false.

The research design is usually incorporated into the introduction of your paper . You can obtain an overall sense of what to do by reviewing studies that have utilized the same research design [e.g., using a case study approach]. This can help you develop an outline to follow for your own paper.

NOTE : Use the SAGE Research Methods Online and Cases and the SAGE Research Methods Videos databases to search for scholarly resources on how to apply specific research designs and methods . The Research Methods Online database contains links to more than 175,000 pages of SAGE publisher's book, journal, and reference content on quantitative, qualitative, and mixed research methodologies. Also included is a collection of case studies of social research projects that can be used to help you better understand abstract or complex methodological concepts. The Research Methods Videos database contains hours of tutorials, interviews, video case studies, and mini-documentaries covering the entire research process.

Creswell, John W. and J. David Creswell. Research Design: Qualitative, Quantitative, and Mixed Methods Approaches . 5th edition. Thousand Oaks, CA: Sage, 2018; De Vaus, D. A. Research Design in Social Research . London: SAGE, 2001; Gorard, Stephen. Research Design: Creating Robust Approaches for the Social Sciences . Thousand Oaks, CA: Sage, 2013; Leedy, Paul D. and Jeanne Ellis Ormrod. Practical Research: Planning and Design . Tenth edition. Boston, MA: Pearson, 2013; Vogt, W. Paul, Dianna C. Gardner, and Lynne M. Haeffele. When to Use What Research Design . New York: Guilford, 2012.

Action Research Design

Definition and Purpose

The essentials of action research design follow a characteristic cycle whereby initially an exploratory stance is adopted, where an understanding of a problem is developed and plans are made for some form of interventionary strategy. Then the intervention is carried out [the "action" in action research] during which time, pertinent observations are collected in various forms. The new interventional strategies are carried out, and this cyclic process repeats, continuing until a sufficient understanding of [or a valid implementation solution for] the problem is achieved. The protocol is iterative or cyclical in nature and is intended to foster deeper understanding of a given situation, starting with conceptualizing and particularizing the problem and moving through several interventions and evaluations.

What do these studies tell you ?

• This is a collaborative and adaptive research design that lends itself to use in work or community situations.
• Design focuses on pragmatic and solution-driven research outcomes rather than testing theories.
• When practitioners use action research, it has the potential to increase the amount they learn consciously from their experience; the action research cycle can be regarded as a learning cycle.
• Action research studies often have direct and obvious relevance to improving practice and advocating for change.
• There are no hidden controls or preemption of direction by the researcher.

What these studies don't tell you ?

• It is harder to do than conducting conventional research because the researcher takes on responsibilities of advocating for change as well as for researching the topic.
• Action research is much harder to write up because it is less likely that you can use a standard format to report your findings effectively [i.e., data is often in the form of stories or observation].
• Personal over-involvement of the researcher may bias research results.
• The cyclic nature of action research to achieve its twin outcomes of action [e.g. change] and research [e.g. understanding] is time-consuming and complex to conduct.
• Advocating for change usually requires buy-in from study participants.

Coghlan, David and Mary Brydon-Miller. The Sage Encyclopedia of Action Research . Thousand Oaks, CA:  Sage, 2014; Efron, Sara Efrat and Ruth Ravid. Action Research in Education: A Practical Guide . New York: Guilford, 2013; Gall, Meredith. Educational Research: An Introduction . Chapter 18, Action Research. 8th ed. Boston, MA: Pearson/Allyn and Bacon, 2007; Gorard, Stephen. Research Design: Creating Robust Approaches for the Social Sciences . Thousand Oaks, CA: Sage, 2013; Kemmis, Stephen and Robin McTaggart. “Participatory Action Research.” In Handbook of Qualitative Research . Norman Denzin and Yvonna S. Lincoln, eds. 2nd ed. (Thousand Oaks, CA: SAGE, 2000), pp. 567-605; McNiff, Jean. Writing and Doing Action Research . London: Sage, 2014; Reason, Peter and Hilary Bradbury. Handbook of Action Research: Participative Inquiry and Practice . Thousand Oaks, CA: SAGE, 2001.

Case Study Design

A case study is an in-depth study of a particular research problem rather than a sweeping statistical survey or comprehensive comparative inquiry. It is often used to narrow down a very broad field of research into one or a few easily researchable examples. The case study research design is also useful for testing whether a specific theory and model actually applies to phenomena in the real world. It is a useful design when not much is known about an issue or phenomenon.

• Approach excels at bringing us to an understanding of a complex issue through detailed contextual analysis of a limited number of events or conditions and their relationships.
• A researcher using a case study design can apply a variety of methodologies and rely on a variety of sources to investigate a research problem.
• Design can extend experience or add strength to what is already known through previous research.
• Social scientists, in particular, make wide use of this research design to examine contemporary real-life situations and provide the basis for the application of concepts and theories and the extension of methodologies.
• The design can provide detailed descriptions of specific and rare cases.
• A single or small number of cases offers little basis for establishing reliability or to generalize the findings to a wider population of people, places, or things.
• Intense exposure to the study of a case may bias a researcher's interpretation of the findings.
• Design does not facilitate assessment of cause and effect relationships.
• Vital information may be missing, making the case hard to interpret.
• The case may not be representative or typical of the larger problem being investigated.
• If the criteria for selecting a case is because it represents a very unusual or unique phenomenon or problem for study, then your interpretation of the findings can only apply to that particular case.

Case Studies. Writing@CSU. Colorado State University; Anastas, Jeane W. Research Design for Social Work and the Human Services . Chapter 4, Flexible Methods: Case Study Design. 2nd ed. New York: Columbia University Press, 1999; Gerring, John. “What Is a Case Study and What Is It Good for?” American Political Science Review 98 (May 2004): 341-354; Greenhalgh, Trisha, editor. Case Study Evaluation: Past, Present and Future Challenges . Bingley, UK: Emerald Group Publishing, 2015; Mills, Albert J. , Gabrielle Durepos, and Eiden Wiebe, editors. Encyclopedia of Case Study Research . Thousand Oaks, CA: SAGE Publications, 2010; Stake, Robert E. The Art of Case Study Research . Thousand Oaks, CA: SAGE, 1995; Yin, Robert K. Case Study Research: Design and Theory . Applied Social Research Methods Series, no. 5. 3rd ed. Thousand Oaks, CA: SAGE, 2003.

Causal Design

Causality studies may be thought of as understanding a phenomenon in terms of conditional statements in the form, “If X, then Y.” This type of research is used to measure what impact a specific change will have on existing norms and assumptions. Most social scientists seek causal explanations that reflect tests of hypotheses. Causal effect (nomothetic perspective) occurs when variation in one phenomenon, an independent variable, leads to or results, on average, in variation in another phenomenon, the dependent variable.

Conditions necessary for determining causality:

• Empirical association -- a valid conclusion is based on finding an association between the independent variable and the dependent variable.
• Appropriate time order -- to conclude that causation was involved, one must see that cases were exposed to variation in the independent variable before variation in the dependent variable.
• Nonspuriousness -- a relationship between two variables that is not due to variation in a third variable.
• Causality research designs assist researchers in understanding why the world works the way it does through the process of proving a causal link between variables and by the process of eliminating other possibilities.
• Replication is possible.
• There is greater confidence the study has internal validity due to the systematic subject selection and equity of groups being compared.
• Not all relationships are causal! The possibility always exists that, by sheer coincidence, two unrelated events appear to be related [e.g., Punxatawney Phil could accurately predict the duration of Winter for five consecutive years but, the fact remains, he's just a big, furry rodent].
• Conclusions about causal relationships are difficult to determine due to a variety of extraneous and confounding variables that exist in a social environment. This means causality can only be inferred, never proven.
• If two variables are correlated, the cause must come before the effect. However, even though two variables might be causally related, it can sometimes be difficult to determine which variable comes first and, therefore, to establish which variable is the actual cause and which is the  actual effect.

Beach, Derek and Rasmus Brun Pedersen. Causal Case Study Methods: Foundations and Guidelines for Comparing, Matching, and Tracing . Ann Arbor, MI: University of Michigan Press, 2016; Bachman, Ronet. The Practice of Research in Criminology and Criminal Justice . Chapter 5, Causation and Research Designs. 3rd ed. Thousand Oaks, CA: Pine Forge Press, 2007; Brewer, Ernest W. and Jennifer Kubn. “Causal-Comparative Design.” In Encyclopedia of Research Design . Neil J. Salkind, editor. (Thousand Oaks, CA: Sage, 2010), pp. 125-132; Causal Research Design: Experimentation. Anonymous SlideShare Presentation; Gall, Meredith. Educational Research: An Introduction . Chapter 11, Nonexperimental Research: Correlational Designs. 8th ed. Boston, MA: Pearson/Allyn and Bacon, 2007; Trochim, William M.K. Research Methods Knowledge Base. 2006.

Cohort Design

Often used in the medical sciences, but also found in the applied social sciences, a cohort study generally refers to a study conducted over a period of time involving members of a population which the subject or representative member comes from, and who are united by some commonality or similarity. Using a quantitative framework, a cohort study makes note of statistical occurrence within a specialized subgroup, united by same or similar characteristics that are relevant to the research problem being investigated, rather than studying statistical occurrence within the general population. Using a qualitative framework, cohort studies generally gather data using methods of observation. Cohorts can be either "open" or "closed."

• Open Cohort Studies [dynamic populations, such as the population of Los Angeles] involve a population that is defined just by the state of being a part of the study in question (and being monitored for the outcome). Date of entry and exit from the study is individually defined, therefore, the size of the study population is not constant. In open cohort studies, researchers can only calculate rate based data, such as, incidence rates and variants thereof.
• Closed Cohort Studies [static populations, such as patients entered into a clinical trial] involve participants who enter into the study at one defining point in time and where it is presumed that no new participants can enter the cohort. Given this, the number of study participants remains constant (or can only decrease).
• The use of cohorts is often mandatory because a randomized control study may be unethical. For example, you cannot deliberately expose people to asbestos, you can only study its effects on those who have already been exposed. Research that measures risk factors often relies upon cohort designs.
• Because cohort studies measure potential causes before the outcome has occurred, they can demonstrate that these “causes” preceded the outcome, thereby avoiding the debate as to which is the cause and which is the effect.
• Cohort analysis is highly flexible and can provide insight into effects over time and related to a variety of different types of changes [e.g., social, cultural, political, economic, etc.].
• Either original data or secondary data can be used in this design.
• In cases where a comparative analysis of two cohorts is made [e.g., studying the effects of one group exposed to asbestos and one that has not], a researcher cannot control for all other factors that might differ between the two groups. These factors are known as confounding variables.
• Cohort studies can end up taking a long time to complete if the researcher must wait for the conditions of interest to develop within the group. This also increases the chance that key variables change during the course of the study, potentially impacting the validity of the findings.
• Due to the lack of randominization in the cohort design, its external validity is lower than that of study designs where the researcher randomly assigns participants.

Healy P, Devane D. “Methodological Considerations in Cohort Study Designs.” Nurse Researcher 18 (2011): 32-36; Glenn, Norval D, editor. Cohort Analysis . 2nd edition. Thousand Oaks, CA: Sage, 2005; Levin, Kate Ann. Study Design IV: Cohort Studies. Evidence-Based Dentistry 7 (2003): 51–52; Payne, Geoff. “Cohort Study.” In The SAGE Dictionary of Social Research Methods . Victor Jupp, editor. (Thousand Oaks, CA: Sage, 2006), pp. 31-33; Study Design 101. Himmelfarb Health Sciences Library. George Washington University, November 2011; Cohort Study. Wikipedia.

Cross-Sectional Design

Cross-sectional research designs have three distinctive features: no time dimension; a reliance on existing differences rather than change following intervention; and, groups are selected based on existing differences rather than random allocation. The cross-sectional design can only measure differences between or from among a variety of people, subjects, or phenomena rather than a process of change. As such, researchers using this design can only employ a relatively passive approach to making causal inferences based on findings.

• Cross-sectional studies provide a clear 'snapshot' of the outcome and the characteristics associated with it, at a specific point in time.
• Unlike an experimental design, where there is an active intervention by the researcher to produce and measure change or to create differences, cross-sectional designs focus on studying and drawing inferences from existing differences between people, subjects, or phenomena.
• Entails collecting data at and concerning one point in time. While longitudinal studies involve taking multiple measures over an extended period of time, cross-sectional research is focused on finding relationships between variables at one moment in time.
• Groups identified for study are purposely selected based upon existing differences in the sample rather than seeking random sampling.
• Cross-section studies are capable of using data from a large number of subjects and, unlike observational studies, is not geographically bound.
• Can estimate prevalence of an outcome of interest because the sample is usually taken from the whole population.
• Because cross-sectional designs generally use survey techniques to gather data, they are relatively inexpensive and take up little time to conduct.
• Finding people, subjects, or phenomena to study that are very similar except in one specific variable can be difficult.
• Results are static and time bound and, therefore, give no indication of a sequence of events or reveal historical or temporal contexts.
• Studies cannot be utilized to establish cause and effect relationships.
• This design only provides a snapshot of analysis so there is always the possibility that a study could have differing results if another time-frame had been chosen.
• There is no follow up to the findings.

Bethlehem, Jelke. "7: Cross-sectional Research." In Research Methodology in the Social, Behavioural and Life Sciences . Herman J Adèr and Gideon J Mellenbergh, editors. (London, England: Sage, 1999), pp. 110-43; Bourque, Linda B. “Cross-Sectional Design.” In  The SAGE Encyclopedia of Social Science Research Methods . Michael S. Lewis-Beck, Alan Bryman, and Tim Futing Liao. (Thousand Oaks, CA: 2004), pp. 230-231; Hall, John. “Cross-Sectional Survey Design.” In Encyclopedia of Survey Research Methods . Paul J. Lavrakas, ed. (Thousand Oaks, CA: Sage, 2008), pp. 173-174; Helen Barratt, Maria Kirwan. Cross-Sectional Studies: Design Application, Strengths and Weaknesses of Cross-Sectional Studies. Healthknowledge, 2009. Cross-Sectional Study. Wikipedia.

Descriptive Design

Descriptive research designs help provide answers to the questions of who, what, when, where, and how associated with a particular research problem; a descriptive study cannot conclusively ascertain answers to why. Descriptive research is used to obtain information concerning the current status of the phenomena and to describe "what exists" with respect to variables or conditions in a situation.

• The subject is being observed in a completely natural and unchanged natural environment. True experiments, whilst giving analyzable data, often adversely influence the normal behavior of the subject [a.k.a., the Heisenberg effect whereby measurements of certain systems cannot be made without affecting the systems].
• Descriptive research is often used as a pre-cursor to more quantitative research designs with the general overview giving some valuable pointers as to what variables are worth testing quantitatively.
• If the limitations are understood, they can be a useful tool in developing a more focused study.
• Descriptive studies can yield rich data that lead to important recommendations in practice.
• Appoach collects a large amount of data for detailed analysis.
• The results from a descriptive research cannot be used to discover a definitive answer or to disprove a hypothesis.
• Because descriptive designs often utilize observational methods [as opposed to quantitative methods], the results cannot be replicated.
• The descriptive function of research is heavily dependent on instrumentation for measurement and observation.

Anastas, Jeane W. Research Design for Social Work and the Human Services . Chapter 5, Flexible Methods: Descriptive Research. 2nd ed. New York: Columbia University Press, 1999; Given, Lisa M. "Descriptive Research." In Encyclopedia of Measurement and Statistics . Neil J. Salkind and Kristin Rasmussen, editors. (Thousand Oaks, CA: Sage, 2007), pp. 251-254; McNabb, Connie. Descriptive Research Methodologies. Powerpoint Presentation; Shuttleworth, Martyn. Descriptive Research Design, September 26, 2008; Erickson, G. Scott. "Descriptive Research Design." In New Methods of Market Research and Analysis . (Northampton, MA: Edward Elgar Publishing, 2017), pp. 51-77; Sahin, Sagufta, and Jayanta Mete. "A Brief Study on Descriptive Research: Its Nature and Application in Social Science." International Journal of Research and Analysis in Humanities 1 (2021): 11; K. Swatzell and P. Jennings. “Descriptive Research: The Nuts and Bolts.” Journal of the American Academy of Physician Assistants 20 (2007), pp. 55-56; Kane, E. Doing Your Own Research: Basic Descriptive Research in the Social Sciences and Humanities . London: Marion Boyars, 1985.

Experimental Design

A blueprint of the procedure that enables the researcher to maintain control over all factors that may affect the result of an experiment. In doing this, the researcher attempts to determine or predict what may occur. Experimental research is often used where there is time priority in a causal relationship (cause precedes effect), there is consistency in a causal relationship (a cause will always lead to the same effect), and the magnitude of the correlation is great. The classic experimental design specifies an experimental group and a control group. The independent variable is administered to the experimental group and not to the control group, and both groups are measured on the same dependent variable. Subsequent experimental designs have used more groups and more measurements over longer periods. True experiments must have control, randomization, and manipulation.

• Experimental research allows the researcher to control the situation. In so doing, it allows researchers to answer the question, “What causes something to occur?”
• Permits the researcher to identify cause and effect relationships between variables and to distinguish placebo effects from treatment effects.
• Experimental research designs support the ability to limit alternative explanations and to infer direct causal relationships in the study.
• Approach provides the highest level of evidence for single studies.
• The design is artificial, and results may not generalize well to the real world.
• The artificial settings of experiments may alter the behaviors or responses of participants.
• Experimental designs can be costly if special equipment or facilities are needed.
• Some research problems cannot be studied using an experiment because of ethical or technical reasons.
• Difficult to apply ethnographic and other qualitative methods to experimentally designed studies.

Anastas, Jeane W. Research Design for Social Work and the Human Services . Chapter 7, Flexible Methods: Experimental Research. 2nd ed. New York: Columbia University Press, 1999; Chapter 2: Research Design, Experimental Designs. School of Psychology, University of New England, 2000; Chow, Siu L. "Experimental Design." In Encyclopedia of Research Design . Neil J. Salkind, editor. (Thousand Oaks, CA: Sage, 2010), pp. 448-453; "Experimental Design." In Social Research Methods . Nicholas Walliman, editor. (London, England: Sage, 2006), pp, 101-110; Experimental Research. Research Methods by Dummies. Department of Psychology. California State University, Fresno, 2006; Kirk, Roger E. Experimental Design: Procedures for the Behavioral Sciences . 4th edition. Thousand Oaks, CA: Sage, 2013; Trochim, William M.K. Experimental Design. Research Methods Knowledge Base. 2006; Rasool, Shafqat. Experimental Research. Slideshare presentation.

Exploratory Design

An exploratory design is conducted about a research problem when there are few or no earlier studies to refer to or rely upon to predict an outcome . The focus is on gaining insights and familiarity for later investigation or undertaken when research problems are in a preliminary stage of investigation. Exploratory designs are often used to establish an understanding of how best to proceed in studying an issue or what methodology would effectively apply to gathering information about the issue.

The goals of exploratory research are intended to produce the following possible insights:

• Familiarity with basic details, settings, and concerns.
• Well grounded picture of the situation being developed.
• Generation of new ideas and assumptions.
• Development of tentative theories or hypotheses.
• Determination about whether a study is feasible in the future.
• Issues get refined for more systematic investigation and formulation of new research questions.
• Direction for future research and techniques get developed.
• Design is a useful approach for gaining background information on a particular topic.
• Exploratory research is flexible and can address research questions of all types (what, why, how).
• Provides an opportunity to define new terms and clarify existing concepts.
• Exploratory research is often used to generate formal hypotheses and develop more precise research problems.
• In the policy arena or applied to practice, exploratory studies help establish research priorities and where resources should be allocated.
• Exploratory research generally utilizes small sample sizes and, thus, findings are typically not generalizable to the population at large.
• The exploratory nature of the research inhibits an ability to make definitive conclusions about the findings. They provide insight but not definitive conclusions.
• The research process underpinning exploratory studies is flexible but often unstructured, leading to only tentative results that have limited value to decision-makers.
• Design lacks rigorous standards applied to methods of data gathering and analysis because one of the areas for exploration could be to determine what method or methodologies could best fit the research problem.

Cuthill, Michael. “Exploratory Research: Citizen Participation, Local Government, and Sustainable Development in Australia.” Sustainable Development 10 (2002): 79-89; Streb, Christoph K. "Exploratory Case Study." In Encyclopedia of Case Study Research . Albert J. Mills, Gabrielle Durepos and Eiden Wiebe, editors. (Thousand Oaks, CA: Sage, 2010), pp. 372-374; Taylor, P. J., G. Catalano, and D.R.F. Walker. “Exploratory Analysis of the World City Network.” Urban Studies 39 (December 2002): 2377-2394; Exploratory Research. Wikipedia.

Field Research Design

Sometimes referred to as ethnography or participant observation, designs around field research encompass a variety of interpretative procedures [e.g., observation and interviews] rooted in qualitative approaches to studying people individually or in groups while inhabiting their natural environment as opposed to using survey instruments or other forms of impersonal methods of data gathering. Information acquired from observational research takes the form of “ field notes ” that involves documenting what the researcher actually sees and hears while in the field. Findings do not consist of conclusive statements derived from numbers and statistics because field research involves analysis of words and observations of behavior. Conclusions, therefore, are developed from an interpretation of findings that reveal overriding themes, concepts, and ideas. More information can be found HERE .

• Field research is often necessary to fill gaps in understanding the research problem applied to local conditions or to specific groups of people that cannot be ascertained from existing data.
• The research helps contextualize already known information about a research problem, thereby facilitating ways to assess the origins, scope, and scale of a problem and to gage the causes, consequences, and means to resolve an issue based on deliberate interaction with people in their natural inhabited spaces.
• Enables the researcher to corroborate or confirm data by gathering additional information that supports or refutes findings reported in prior studies of the topic.
• Because the researcher in embedded in the field, they are better able to make observations or ask questions that reflect the specific cultural context of the setting being investigated.
• Observing the local reality offers the opportunity to gain new perspectives or obtain unique data that challenges existing theoretical propositions or long-standing assumptions found in the literature.

What these studies don't tell you

• A field research study requires extensive time and resources to carry out the multiple steps involved with preparing for the gathering of information, including for example, examining background information about the study site, obtaining permission to access the study site, and building trust and rapport with subjects.
• Requires a commitment to staying engaged in the field to ensure that you can adequately document events and behaviors as they unfold.
• The unpredictable nature of fieldwork means that researchers can never fully control the process of data gathering. They must maintain a flexible approach to studying the setting because events and circumstances can change quickly or unexpectedly.
• Findings can be difficult to interpret and verify without access to documents and other source materials that help to enhance the credibility of information obtained from the field  [i.e., the act of triangulating the data].
• Linking the research problem to the selection of study participants inhabiting their natural environment is critical. However, this specificity limits the ability to generalize findings to different situations or in other contexts or to infer courses of action applied to other settings or groups of people.
• The reporting of findings must take into account how the researcher themselves may have inadvertently affected respondents and their behaviors.

Historical Design

The purpose of a historical research design is to collect, verify, and synthesize evidence from the past to establish facts that defend or refute a hypothesis. It uses secondary sources and a variety of primary documentary evidence, such as, diaries, official records, reports, archives, and non-textual information [maps, pictures, audio and visual recordings]. The limitation is that the sources must be both authentic and valid.

• The historical research design is unobtrusive; the act of research does not affect the results of the study.
• The historical approach is well suited for trend analysis.
• Historical records can add important contextual background required to more fully understand and interpret a research problem.
• There is often no possibility of researcher-subject interaction that could affect the findings.
• Historical sources can be used over and over to study different research problems or to replicate a previous study.
• The ability to fulfill the aims of your research are directly related to the amount and quality of documentation available to understand the research problem.
• Since historical research relies on data from the past, there is no way to manipulate it to control for contemporary contexts.
• Interpreting historical sources can be very time consuming.
• The sources of historical materials must be archived consistently to ensure access. This may especially challenging for digital or online-only sources.
• Original authors bring their own perspectives and biases to the interpretation of past events and these biases are more difficult to ascertain in historical resources.
• Due to the lack of control over external variables, historical research is very weak with regard to the demands of internal validity.
• It is rare that the entirety of historical documentation needed to fully address a research problem is available for interpretation, therefore, gaps need to be acknowledged.

Howell, Martha C. and Walter Prevenier. From Reliable Sources: An Introduction to Historical Methods . Ithaca, NY: Cornell University Press, 2001; Lundy, Karen Saucier. "Historical Research." In The Sage Encyclopedia of Qualitative Research Methods . Lisa M. Given, editor. (Thousand Oaks, CA: Sage, 2008), pp. 396-400; Marius, Richard. and Melvin E. Page. A Short Guide to Writing about History . 9th edition. Boston, MA: Pearson, 2015; Savitt, Ronald. “Historical Research in Marketing.” Journal of Marketing 44 (Autumn, 1980): 52-58;  Gall, Meredith. Educational Research: An Introduction . Chapter 16, Historical Research. 8th ed. Boston, MA: Pearson/Allyn and Bacon, 2007.

Longitudinal Design

A longitudinal study follows the same sample over time and makes repeated observations. For example, with longitudinal surveys, the same group of people is interviewed at regular intervals, enabling researchers to track changes over time and to relate them to variables that might explain why the changes occur. Longitudinal research designs describe patterns of change and help establish the direction and magnitude of causal relationships. Measurements are taken on each variable over two or more distinct time periods. This allows the researcher to measure change in variables over time. It is a type of observational study sometimes referred to as a panel study.

• Longitudinal data facilitate the analysis of the duration of a particular phenomenon.
• Enables survey researchers to get close to the kinds of causal explanations usually attainable only with experiments.
• The design permits the measurement of differences or change in a variable from one period to another [i.e., the description of patterns of change over time].
• Longitudinal studies facilitate the prediction of future outcomes based upon earlier factors.
• The data collection method may change over time.
• Maintaining the integrity of the original sample can be difficult over an extended period of time.
• It can be difficult to show more than one variable at a time.
• This design often needs qualitative research data to explain fluctuations in the results.
• A longitudinal research design assumes present trends will continue unchanged.
• It can take a long period of time to gather results.
• There is a need to have a large sample size and accurate sampling to reach representativness.

Anastas, Jeane W. Research Design for Social Work and the Human Services . Chapter 6, Flexible Methods: Relational and Longitudinal Research. 2nd ed. New York: Columbia University Press, 1999; Forgues, Bernard, and Isabelle Vandangeon-Derumez. "Longitudinal Analyses." In Doing Management Research . Raymond-Alain Thiétart and Samantha Wauchope, editors. (London, England: Sage, 2001), pp. 332-351; Kalaian, Sema A. and Rafa M. Kasim. "Longitudinal Studies." In Encyclopedia of Survey Research Methods . Paul J. Lavrakas, ed. (Thousand Oaks, CA: Sage, 2008), pp. 440-441; Menard, Scott, editor. Longitudinal Research . Thousand Oaks, CA: Sage, 2002; Ployhart, Robert E. and Robert J. Vandenberg. "Longitudinal Research: The Theory, Design, and Analysis of Change.” Journal of Management 36 (January 2010): 94-120; Longitudinal Study. Wikipedia.

Meta-Analysis Design

Meta-analysis is an analytical methodology designed to systematically evaluate and summarize the results from a number of individual studies, thereby, increasing the overall sample size and the ability of the researcher to study effects of interest. The purpose is to not simply summarize existing knowledge, but to develop a new understanding of a research problem using synoptic reasoning. The main objectives of meta-analysis include analyzing differences in the results among studies and increasing the precision by which effects are estimated. A well-designed meta-analysis depends upon strict adherence to the criteria used for selecting studies and the availability of information in each study to properly analyze their findings. Lack of information can severely limit the type of analyzes and conclusions that can be reached. In addition, the more dissimilarity there is in the results among individual studies [heterogeneity], the more difficult it is to justify interpretations that govern a valid synopsis of results. A meta-analysis needs to fulfill the following requirements to ensure the validity of your findings:

• Clearly defined description of objectives, including precise definitions of the variables and outcomes that are being evaluated;
• A well-reasoned and well-documented justification for identification and selection of the studies;
• Assessment and explicit acknowledgment of any researcher bias in the identification and selection of those studies;
• Description and evaluation of the degree of heterogeneity among the sample size of studies reviewed; and,
• Justification of the techniques used to evaluate the studies.
• Can be an effective strategy for determining gaps in the literature.
• Provides a means of reviewing research published about a particular topic over an extended period of time and from a variety of sources.
• Is useful in clarifying what policy or programmatic actions can be justified on the basis of analyzing research results from multiple studies.
• Provides a method for overcoming small sample sizes in individual studies that previously may have had little relationship to each other.
• Can be used to generate new hypotheses or highlight research problems for future studies.
• Small violations in defining the criteria used for content analysis can lead to difficult to interpret and/or meaningless findings.
• A large sample size can yield reliable, but not necessarily valid, results.
• A lack of uniformity regarding, for example, the type of literature reviewed, how methods are applied, and how findings are measured within the sample of studies you are analyzing, can make the process of synthesis difficult to perform.
• Depending on the sample size, the process of reviewing and synthesizing multiple studies can be very time consuming.

Beck, Lewis W. "The Synoptic Method." The Journal of Philosophy 36 (1939): 337-345; Cooper, Harris, Larry V. Hedges, and Jeffrey C. Valentine, eds. The Handbook of Research Synthesis and Meta-Analysis . 2nd edition. New York: Russell Sage Foundation, 2009; Guzzo, Richard A., Susan E. Jackson and Raymond A. Katzell. “Meta-Analysis Analysis.” In Research in Organizational Behavior , Volume 9. (Greenwich, CT: JAI Press, 1987), pp 407-442; Lipsey, Mark W. and David B. Wilson. Practical Meta-Analysis . Thousand Oaks, CA: Sage Publications, 2001; Study Design 101. Meta-Analysis. The Himmelfarb Health Sciences Library, George Washington University; Timulak, Ladislav. “Qualitative Meta-Analysis.” In The SAGE Handbook of Qualitative Data Analysis . Uwe Flick, editor. (Los Angeles, CA: Sage, 2013), pp. 481-495; Walker, Esteban, Adrian V. Hernandez, and Micheal W. Kattan. "Meta-Analysis: It's Strengths and Limitations." Cleveland Clinic Journal of Medicine 75 (June 2008): 431-439.

Mixed-Method Design

• Narrative and non-textual information can add meaning to numeric data, while numeric data can add precision to narrative and non-textual information.
• Can utilize existing data while at the same time generating and testing a grounded theory approach to describe and explain the phenomenon under study.
• A broader, more complex research problem can be investigated because the researcher is not constrained by using only one method.
• The strengths of one method can be used to overcome the inherent weaknesses of another method.
• Can provide stronger, more robust evidence to support a conclusion or set of recommendations.
• May generate new knowledge new insights or uncover hidden insights, patterns, or relationships that a single methodological approach might not reveal.
• Produces more complete knowledge and understanding of the research problem that can be used to increase the generalizability of findings applied to theory or practice.
• A researcher must be proficient in understanding how to apply multiple methods to investigating a research problem as well as be proficient in optimizing how to design a study that coherently melds them together.
• Can increase the likelihood of conflicting results or ambiguous findings that inhibit drawing a valid conclusion or setting forth a recommended course of action [e.g., sample interview responses do not support existing statistical data].
• Because the research design can be very complex, reporting the findings requires a well-organized narrative, clear writing style, and precise word choice.
• Design invites collaboration among experts. However, merging different investigative approaches and writing styles requires more attention to the overall research process than studies conducted using only one methodological paradigm.
• Concurrent merging of quantitative and qualitative research requires greater attention to having adequate sample sizes, using comparable samples, and applying a consistent unit of analysis. For sequential designs where one phase of qualitative research builds on the quantitative phase or vice versa, decisions about what results from the first phase to use in the next phase, the choice of samples and estimating reasonable sample sizes for both phases, and the interpretation of results from both phases can be difficult.
• Due to multiple forms of data being collected and analyzed, this design requires extensive time and resources to carry out the multiple steps involved in data gathering and interpretation.

Burch, Patricia and Carolyn J. Heinrich. Mixed Methods for Policy Research and Program Evaluation . Thousand Oaks, CA: Sage, 2016; Creswell, John w. et al. Best Practices for Mixed Methods Research in the Health Sciences . Bethesda, MD: Office of Behavioral and Social Sciences Research, National Institutes of Health, 2010Creswell, John W. Research Design: Qualitative, Quantitative, and Mixed Methods Approaches . 4th edition. Thousand Oaks, CA: Sage Publications, 2014; Domínguez, Silvia, editor. Mixed Methods Social Networks Research . Cambridge, UK: Cambridge University Press, 2014; Hesse-Biber, Sharlene Nagy. Mixed Methods Research: Merging Theory with Practice . New York: Guilford Press, 2010; Niglas, Katrin. “How the Novice Researcher Can Make Sense of Mixed Methods Designs.” International Journal of Multiple Research Approaches 3 (2009): 34-46; Onwuegbuzie, Anthony J. and Nancy L. Leech. “Linking Research Questions to Mixed Methods Data Analysis Procedures.” The Qualitative Report 11 (September 2006): 474-498; Tashakorri, Abbas and John W. Creswell. “The New Era of Mixed Methods.” Journal of Mixed Methods Research 1 (January 2007): 3-7; Zhanga, Wanqing. “Mixed Methods Application in Health Intervention Research: A Multiple Case Study.” International Journal of Multiple Research Approaches 8 (2014): 24-35 .

Observational Design

This type of research design draws a conclusion by comparing subjects against a control group, in cases where the researcher has no control over the experiment. There are two general types of observational designs. In direct observations, people know that you are watching them. Unobtrusive measures involve any method for studying behavior where individuals do not know they are being observed. An observational study allows a useful insight into a phenomenon and avoids the ethical and practical difficulties of setting up a large and cumbersome research project.

• Observational studies are usually flexible and do not necessarily need to be structured around a hypothesis about what you expect to observe [data is emergent rather than pre-existing].
• The researcher is able to collect in-depth information about a particular behavior.
• Can reveal interrelationships among multifaceted dimensions of group interactions.
• You can generalize your results to real life situations.
• Observational research is useful for discovering what variables may be important before applying other methods like experiments.
• Observation research designs account for the complexity of group behaviors.
• Reliability of data is low because seeing behaviors occur over and over again may be a time consuming task and are difficult to replicate.
• In observational research, findings may only reflect a unique sample population and, thus, cannot be generalized to other groups.
• There can be problems with bias as the researcher may only "see what they want to see."
• There is no possibility to determine "cause and effect" relationships since nothing is manipulated.
• Sources or subjects may not all be equally credible.
• Any group that is knowingly studied is altered to some degree by the presence of the researcher, therefore, potentially skewing any data collected.

Atkinson, Paul and Martyn Hammersley. “Ethnography and Participant Observation.” In Handbook of Qualitative Research . Norman K. Denzin and Yvonna S. Lincoln, eds. (Thousand Oaks, CA: Sage, 1994), pp. 248-261; Observational Research. Research Methods by Dummies. Department of Psychology. California State University, Fresno, 2006; Patton Michael Quinn. Qualitiative Research and Evaluation Methods . Chapter 6, Fieldwork Strategies and Observational Methods. 3rd ed. Thousand Oaks, CA: Sage, 2002; Payne, Geoff and Judy Payne. "Observation." In Key Concepts in Social Research . The SAGE Key Concepts series. (London, England: Sage, 2004), pp. 158-162; Rosenbaum, Paul R. Design of Observational Studies . New York: Springer, 2010;Williams, J. Patrick. "Nonparticipant Observation." In The Sage Encyclopedia of Qualitative Research Methods . Lisa M. Given, editor.(Thousand Oaks, CA: Sage, 2008), pp. 562-563.

Philosophical Design

Understood more as an broad approach to examining a research problem than a methodological design, philosophical analysis and argumentation is intended to challenge deeply embedded, often intractable, assumptions underpinning an area of study. This approach uses the tools of argumentation derived from philosophical traditions, concepts, models, and theories to critically explore and challenge, for example, the relevance of logic and evidence in academic debates, to analyze arguments about fundamental issues, or to discuss the root of existing discourse about a research problem. These overarching tools of analysis can be framed in three ways:

• Ontology -- the study that describes the nature of reality; for example, what is real and what is not, what is fundamental and what is derivative?
• Epistemology -- the study that explores the nature of knowledge; for example, by what means does knowledge and understanding depend upon and how can we be certain of what we know?
• Axiology -- the study of values; for example, what values does an individual or group hold and why? How are values related to interest, desire, will, experience, and means-to-end? And, what is the difference between a matter of fact and a matter of value?
• Can provide a basis for applying ethical decision-making to practice.
• Functions as a means of gaining greater self-understanding and self-knowledge about the purposes of research.
• Brings clarity to general guiding practices and principles of an individual or group.
• Philosophy informs methodology.
• Refine concepts and theories that are invoked in relatively unreflective modes of thought and discourse.
• Beyond methodology, philosophy also informs critical thinking about epistemology and the structure of reality (metaphysics).
• Offers clarity and definition to the practical and theoretical uses of terms, concepts, and ideas.
• Limited application to specific research problems [answering the "So What?" question in social science research].
• Analysis can be abstract, argumentative, and limited in its practical application to real-life issues.
• While a philosophical analysis may render problematic that which was once simple or taken-for-granted, the writing can be dense and subject to unnecessary jargon, overstatement, and/or excessive quotation and documentation.
• There are limitations in the use of metaphor as a vehicle of philosophical analysis.
• There can be analytical difficulties in moving from philosophy to advocacy and between abstract thought and application to the phenomenal world.

Burton, Dawn. "Part I, Philosophy of the Social Sciences." In Research Training for Social Scientists . (London, England: Sage, 2000), pp. 1-5; Chapter 4, Research Methodology and Design. Unisa Institutional Repository (UnisaIR), University of South Africa; Jarvie, Ian C., and Jesús Zamora-Bonilla, editors. The SAGE Handbook of the Philosophy of Social Sciences . London: Sage, 2011; Labaree, Robert V. and Ross Scimeca. “The Philosophical Problem of Truth in Librarianship.” The Library Quarterly 78 (January 2008): 43-70; Maykut, Pamela S. Beginning Qualitative Research: A Philosophic and Practical Guide . Washington, DC: Falmer Press, 1994; McLaughlin, Hugh. "The Philosophy of Social Research." In Understanding Social Work Research . 2nd edition. (London: SAGE Publications Ltd., 2012), pp. 24-47; Stanford Encyclopedia of Philosophy . Metaphysics Research Lab, CSLI, Stanford University, 2013.

Sequential Design

• The researcher has a limitless option when it comes to sample size and the sampling schedule.
• Due to the repetitive nature of this research design, minor changes and adjustments can be done during the initial parts of the study to correct and hone the research method.
• This is a useful design for exploratory studies.
• There is very little effort on the part of the researcher when performing this technique. It is generally not expensive, time consuming, or workforce intensive.
• Because the study is conducted serially, the results of one sample are known before the next sample is taken and analyzed. This provides opportunities for continuous improvement of sampling and methods of analysis.
• The sampling method is not representative of the entire population. The only possibility of approaching representativeness is when the researcher chooses to use a very large sample size significant enough to represent a significant portion of the entire population. In this case, moving on to study a second or more specific sample can be difficult.
• The design cannot be used to create conclusions and interpretations that pertain to an entire population because the sampling technique is not randomized. Generalizability from findings is, therefore, limited.
• Difficult to account for and interpret variation from one sample to another over time, particularly when using qualitative methods of data collection.

Betensky, Rebecca. Harvard University, Course Lecture Note slides; Bovaird, James A. and Kevin A. Kupzyk. "Sequential Design." In Encyclopedia of Research Design . Neil J. Salkind, editor. (Thousand Oaks, CA: Sage, 2010), pp. 1347-1352; Cresswell, John W. Et al. “Advanced Mixed-Methods Research Designs.” In Handbook of Mixed Methods in Social and Behavioral Research . Abbas Tashakkori and Charles Teddle, eds. (Thousand Oaks, CA: Sage, 2003), pp. 209-240; Henry, Gary T. "Sequential Sampling." In The SAGE Encyclopedia of Social Science Research Methods . Michael S. Lewis-Beck, Alan Bryman and Tim Futing Liao, editors. (Thousand Oaks, CA: Sage, 2004), pp. 1027-1028; Nataliya V. Ivankova. “Using Mixed-Methods Sequential Explanatory Design: From Theory to Practice.” Field Methods 18 (February 2006): 3-20; Bovaird, James A. and Kevin A. Kupzyk. “Sequential Design.” In Encyclopedia of Research Design . Neil J. Salkind, ed. Thousand Oaks, CA: Sage, 2010; Sequential Analysis. Wikipedia.

Systematic Review

• A systematic review synthesizes the findings of multiple studies related to each other by incorporating strategies of analysis and interpretation intended to reduce biases and random errors.
• The application of critical exploration, evaluation, and synthesis methods separates insignificant, unsound, or redundant research from the most salient and relevant studies worthy of reflection.
• They can be use to identify, justify, and refine hypotheses, recognize and avoid hidden problems in prior studies, and explain data inconsistencies and conflicts in data.
• Systematic reviews can be used to help policy makers formulate evidence-based guidelines and regulations.
• The use of strict, explicit, and pre-determined methods of synthesis, when applied appropriately, provide reliable estimates about the effects of interventions, evaluations, and effects related to the overarching research problem investigated by each study under review.
• Systematic reviews illuminate where knowledge or thorough understanding of a research problem is lacking and, therefore, can then be used to guide future research.
• The accepted inclusion of unpublished studies [i.e., grey literature] ensures the broadest possible way to analyze and interpret research on a topic.
• Results of the synthesis can be generalized and the findings extrapolated into the general population with more validity than most other types of studies .
• Systematic reviews do not create new knowledge per se; they are a method for synthesizing existing studies about a research problem in order to gain new insights and determine gaps in the literature.
• The way researchers have carried out their investigations [e.g., the period of time covered, number of participants, sources of data analyzed, etc.] can make it difficult to effectively synthesize studies.
• The inclusion of unpublished studies can introduce bias into the review because they may not have undergone a rigorous peer-review process prior to publication. Examples may include conference presentations or proceedings, publications from government agencies, white papers, working papers, and internal documents from organizations, and doctoral dissertations and Master's theses.

Denyer, David and David Tranfield. "Producing a Systematic Review." In The Sage Handbook of Organizational Research Methods .  David A. Buchanan and Alan Bryman, editors. ( Thousand Oaks, CA: Sage Publications, 2009), pp. 671-689; Foster, Margaret J. and Sarah T. Jewell, editors. Assembling the Pieces of a Systematic Review: A Guide for Librarians . Lanham, MD: Rowman and Littlefield, 2017; Gough, David, Sandy Oliver, James Thomas, editors. Introduction to Systematic Reviews . 2nd edition. Los Angeles, CA: Sage Publications, 2017; Gopalakrishnan, S. and P. Ganeshkumar. “Systematic Reviews and Meta-analysis: Understanding the Best Evidence in Primary Healthcare.” Journal of Family Medicine and Primary Care 2 (2013): 9-14; Gough, David, James Thomas, and Sandy Oliver. "Clarifying Differences between Review Designs and Methods." Systematic Reviews 1 (2012): 1-9; Khan, Khalid S., Regina Kunz, Jos Kleijnen, and Gerd Antes. “Five Steps to Conducting a Systematic Review.” Journal of the Royal Society of Medicine 96 (2003): 118-121; Mulrow, C. D. “Systematic Reviews: Rationale for Systematic Reviews.” BMJ 309:597 (September 1994); O'Dwyer, Linda C., and Q. Eileen Wafford. "Addressing Challenges with Systematic Review Teams through Effective Communication: A Case Report." Journal of the Medical Library Association 109 (October 2021): 643-647; Okoli, Chitu, and Kira Schabram. "A Guide to Conducting a Systematic Literature Review of Information Systems Research."  Sprouts: Working Papers on Information Systems 10 (2010); Siddaway, Andy P., Alex M. Wood, and Larry V. Hedges. "How to Do a Systematic Review: A Best Practice Guide for Conducting and Reporting Narrative Reviews, Meta-analyses, and Meta-syntheses." Annual Review of Psychology 70 (2019): 747-770; Torgerson, Carole J. “Publication Bias: The Achilles’ Heel of Systematic Reviews?” British Journal of Educational Studies 54 (March 2006): 89-102; Torgerson, Carole. Systematic Reviews . New York: Continuum, 2003.

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• Indian J Anaesth
• v.60(9); 2016 Sep

Types of studies and research design

Mukul chandra kapoor.

Department of Anesthesiology, Max Smart Super Specialty Hospital, New Delhi, India

Medical research has evolved, from individual expert described opinions and techniques, to scientifically designed methodology-based studies. Evidence-based medicine (EBM) was established to re-evaluate medical facts and remove various myths in clinical practice. Research methodology is now protocol based with predefined steps. Studies were classified based on the method of collection and evaluation of data. Clinical study methodology now needs to comply to strict ethical, moral, truth, and transparency standards, ensuring that no conflict of interest is involved. A medical research pyramid has been designed to grade the quality of evidence and help physicians determine the value of the research. Randomised controlled trials (RCTs) have become gold standards for quality research. EBM now scales systemic reviews and meta-analyses at a level higher than RCTs to overcome deficiencies in the randomised trials due to errors in methodology and analyses.

INTRODUCTION

Expert opinion, experience, and authoritarian judgement were the norm in clinical medical practice. At scientific meetings, one often heard senior professionals emphatically expressing ‘In my experience,…… what I have said is correct!’ In 1981, articles published by Sackett et al . introduced ‘critical appraisal’ as they felt a need to teach methods of understanding scientific literature and its application at the bedside.[ 1 ] To improve clinical outcomes, clinical expertise must be complemented by the best external evidence.[ 2 ] Conversely, without clinical expertise, good external evidence may be used inappropriately [ Figure 1 ]. Practice gets outdated, if not updated with current evidence, depriving the clientele of the best available therapy.

Triad of evidence-based medicine

EVIDENCE-BASED MEDICINE

In 1971, in his book ‘Effectiveness and Efficiency’, Archibald Cochrane highlighted the lack of reliable evidence behind many accepted health-care interventions.[ 3 ] This triggered re-evaluation of many established ‘supposed’ scientific facts and awakened physicians to the need for evidence in medicine. Evidence-based medicine (EBM) thus evolved, which was defined as ‘the conscientious, explicit and judicious use of the current best evidence in making decisions about the care of individual patients.’[ 2 ]

The goal of EBM was scientific endowment to achieve consistency, efficiency, effectiveness, quality, safety, reduction in dilemma and limitation of idiosyncrasies in clinical practice.[ 4 ] EBM required the physician to diligently assess the therapy, make clinical adjustments using the best available external evidence, ensure awareness of current research and discover clinical pathways to ensure best patient outcomes.[ 5 ]

With widespread internet use, phenomenally large number of publications, training and media resources are available but determining the quality of this literature is difficult for a busy physician. Abstracts are available freely on the internet, but full-text articles require a subscription. To complicate issues, contradictory studies are published making decision-making difficult.[ 6 ] Publication bias, especially against negative studies, makes matters worse.

In 1993, the Cochrane Collaboration was founded by Ian Chalmers and others to create and disseminate up-to-date review of randomised controlled trials (RCTs) to help health-care professionals make informed decisions.[ 7 ] In 1995, the American College of Physicians and the British Medical Journal Publishing Group collaborated to publish the journal ‘Evidence-based medicine’, leading to the evolution of EBM in all spheres of medicine.

MEDICAL RESEARCH

Medical research needs to be conducted to increase knowledge about the human species, its social/natural environment and to combat disease/infirmity in humans. Research should be conducted in a manner conducive to and consistent with dignity and well-being of the participant; in a professional and transparent manner; and ensuring minimal risk.[ 8 ] Research thus must be subjected to careful evaluation at all stages, i.e., research design/experimentation; results and their implications; the objective of the research sought; anticipated benefits/dangers; potential uses/abuses of the experiment and its results; and on ensuring the safety of human life. Table 1 lists the principles any research should follow.[ 8 ]

General principles of medical research

Types of study design

Medical research is classified into primary and secondary research. Clinical/experimental studies are performed in primary research, whereas secondary research consolidates available studies as reviews, systematic reviews and meta-analyses. Three main areas in primary research are basic medical research, clinical research and epidemiological research [ Figure 2 ]. Basic research includes fundamental research in fields shown in Figure 2 . In almost all studies, at least one independent variable is varied, whereas the effects on the dependent variables are investigated. Clinical studies include observational studies and interventional studies and are subclassified as in Figure 2 .

Classification of types of medical research

Interventional clinical study is performed with the purpose of studying or demonstrating clinical or pharmacological properties of drugs/devices, their side effects and to establish their efficacy or safety. They also include studies in which surgical, physical or psychotherapeutic procedures are examined.[ 9 ] Studies on drugs/devices are subject to legal and ethical requirements including the Drug Controller General India (DCGI) directives. They require the approval of DCGI recognized Ethics Committee and must be performed in accordance with the rules of ‘Good Clinical Practice’.[ 10 ] Further details are available under ‘Methodology for research II’ section in this issue of IJA. In 2004, the World Health Organization advised registration of all clinical trials in a public registry. In India, the Clinical Trials Registry of India was launched in 2007 ( www.ctri.nic.in ). The International Committee of Medical Journal Editors (ICMJE) mandates its member journals to publish only registered trials.[ 11 ]

Observational clinical study is a study in which knowledge from treatment of persons with drugs is analysed using epidemiological methods. In these studies, the diagnosis, treatment and monitoring are performed exclusively according to medical practice and not according to a specified study protocol.[ 9 ] They are subclassified as per Figure 2 .

Epidemiological studies have two basic approaches, the interventional and observational. Clinicians are more familiar with interventional research, whereas epidemiologists usually perform observational research.

Interventional studies are experimental in character and are subdivided into field and group studies, for example, iodine supplementation of cooking salt to prevent hypothyroidism. Many interventions are unsuitable for RCTs, as the exposure may be harmful to the subjects.

Observational studies can be subdivided into cohort, case–control, cross-sectional and ecological studies.

• Cohort studies are suited to detect connections between exposure and development of disease. They are normally prospective studies of two healthy groups of subjects observed over time, in which one group is exposed to a specific substance, whereas the other is not. The occurrence of the disease can be determined in the two groups. Cohort studies can also be retrospective
• Case–control studies are retrospective analyses performed to establish the prevalence of a disease in two groups exposed to a factor or disease. The incidence rate cannot be calculated, and there is also a risk of selection bias and faulty recall.

Secondary research

Narrative review.

An expert senior author writes about a particular field, condition or treatment, including an overview, and this information is fortified by his experience. The article is in a narrative format. Its limitation is that one cannot tell whether recommendations are based on author's clinical experience, available literature and why some studies were given more emphasis. It can be biased, with selective citation of reports that reinforce the authors' views of a topic.[ 12 ]

Systematic review

Systematic reviews methodically and comprehensively identify studies focused on a specified topic, appraise their methodology, summate the results, identify key findings and reasons for differences across studies, and cite limitations of current knowledge.[ 13 ] They adhere to reproducible methods and recommended guidelines.[ 14 ] The methods used to compile data are explicit and transparent, allowing the reader to gauge the quality of the review and the potential for bias.[ 15 ]

A systematic review can be presented in text or graphic form. In graphic form, data of different trials can be plotted with the point estimate and 95% confidence interval for each study, presented on an individual line. A properly conducted systematic review presents the best available research evidence for a focused clinical question. The review team may obtain information, not available in the original reports, from the primary authors. This ensures that findings are consistent and generalisable across populations, environment, therapies and groups.[ 12 ] A systematic review attempts to reduce bias identification and studies selection for review, using a comprehensive search strategy and specifying inclusion criteria. The strength of a systematic review lies in the transparency of each phase and highlighting the merits of each decision made, while compiling information.

Meta-analysis

A review team compiles aggregate-level data in each primary study, and in some cases, data are solicited from each of the primary studies.[ 16 , 17 ] Although difficult to perform, individual patient meta-analyses offer advantages over aggregate-level analyses.[ 18 ] These mathematically pooled results are referred to as meta-analysis. Combining data from well-conducted primary studies provide a precise estimate of the “true effect.”[ 19 ] Pooling the samples of individual studies increases overall sample size, enhances statistical analysis power, reduces confidence interval and thereby improves statistical value.

The structured process of Cochrane Collaboration systematic reviews has contributed to the improvement of their quality. For the meta-analysis to be definitive, the primary RCTs should have been conducted methodically. When the existing studies have important scientific and methodological limitations, such as smaller sized samples, the systematic review may identify where gaps exist in the available literature.[ 20 ] RCTs and systematic review of several randomised trials are less likely to mislead us, and thereby help judge whether an intervention is better.[ 2 ] Practice guidelines supported by large RCTs and meta-analyses are considered as ‘gold standard’ in EBM. This issue of IJA is accompanied by an editorial on Importance of EBM on research and practice (Guyat and Sriganesh 471_16).[ 21 ] The EBM pyramid grading the value of different types of research studies is shown in Figure 3 .

The evidence-based medicine pyramid

In the last decade, a number of studies and guidelines brought about path-breaking changes in anaesthesiology and critical care. Some guidelines such as the ‘Surviving Sepsis Guidelines-2004’[ 22 ] were later found to be flawed and biased. A number of large RCTs were rejected as their findings were erroneous. Another classic example is that of ENIGMA-I (Evaluation of Nitrous oxide In the Gas Mixture for Anaesthesia)[ 23 ] which implicated nitrous oxide for poor outcomes, but ENIGMA-II[ 24 , 25 ] conducted later, by the same investigators, declared it as safe. The rise and fall of the ‘tight glucose control’ regimen was similar.[ 26 ]

Although RCTs are considered ‘gold standard’ in research, their status is at crossroads today. RCTs have conflicting interests and thus must be evaluated with careful scrutiny. EBM can promote evidence reflected in RCTs and meta-analyses. However, it cannot promulgate evidence not reflected in RCTs. Flawed RCTs and meta-analyses may bring forth erroneous recommendations. EBM thus should not be restricted to RCTs and meta-analyses but must involve tracking down the best external evidence to answer our clinical questions.

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Scholarly articles are published in print and in online scholarly journals, and are reviewed by experts in a process known as a peer review before they are published. They are written for other scholars or experts. There are several different types of scholarly publications, which vary by discipline. Not all journals publish every type of scholarly article.

Perspective articles

One type of scholarly article is a perspective or opinion article where an expert in the field provides an opinion or perspective on some aspect of the research. Rather than offering a comprehensive review of research, in these articles the author offers their opinion or perspective on a concept, or multiple concepts, from the research. These are often written by more seasoned researchers and tend to be short, usually about 2,000 words.

Book reviews are published in many academic journals and their purpose is to provide a review into recently published scholarly books. These are a great way for early career researchers (ECRs) to hone their academic writing skills .

Clinical papers

In medicine and other types of clinical practice, like clinical psychology, clinical case studies are common. These publications describe details of real cases that involve patients and are typically chosen for publication because they contribute something to existing knowledge in the field. This type of paper can include a discussion of symptoms, diagnosis and treatment of a disease or disorder.

Clinical trial papers are also typical in the field of medicine and describe methods and results of controlled studies usually undertaken with large patient groups that have been randomly assigned to treatment and control groups. For example, for a new vaccine there would be a randomised clinical trial where patients would be randomly placed into two groups: one receiving the vaccine and one receiving a placebo. The study would then examine the results on a number of measures for those two groups.

Review articles

Research review articles provide a critical and comprehensive analysis of existing research on a specific topic. Examples are meta-analyses , systematic reviews and literature reviews . Authors of these articles meticulously report on existing research through summarising and analysing, comparing, identifying common themes and gaps in the knowledge base, and providing directions for future research. Unlike original research papers, these are considered secondary research because the author is discussing other researchers’ work.

These are great articles to find when you are conducting your own research review and are writing your paper. They identify common findings and gaps and can point the reader to a whole wealth of different articles on a topic of interest to them. Research review articles typically are long, ranging anywhere from 8,000 to 20,000 words. Many disciplines publish these review articles, or chapters, in a published handbook. And they are often created by multiple authors  writing collaboratively .

Note : For ECRs, this can be a great opportunity to collaboratively write with more seasoned researchers. That said, y ou may not be at the stage where you are writing a research review article, especially earlier in your career. Publishers and journals often look to seasoned researchers to write these articles because their experience lends itself to a more sophisticated analysis and interpretation of the work that has been done on this topic.  Although you might think that writing a research review article, for example, is an easier task than writing an original research paper, this doesn’t tend to be true. Reviewers and editors hold a very high standard for research review papers and expect them to offer some new contribution in how they present and interpret the review of the research. They are not looking for a summary of the research as much as a critical analysis.

Original articles

Original research papers , or empirical articles , report on original research, as the name suggests. They are usually detailed studies that report research you have conducted that is original. These are classified as primary literature . Generally, these academic articles will include a hypothesis, the context, methods, results and an interpretation or discussion of those results. These publications are typically long, ranging anywhere from 3,000 to 8,000 words and may extend to 12,000 words for some journals. 

Because original research articles weigh heavily when decisions are made about retention, tenure and promotion (RTP), it is very important to write and publish these types of papers, especially as an ECR. These types of articles require a significant investment of time; understanding the structure of these papers is very important. They follow a typical structure, including: 

• The title , which summarises the main idea or ideas of your study. A good title contains the fewest possible words needed to adequately describe the content and/or purpose of your research paper.
• An abstract , which should be a very short, clear and concise summary of the entire paper. An abstract should include enough detail so the reader will know whether or not they wish to read the paper. It should reveal both the purpose and conclusions of the paper.
• The main text, which includes an introduction , background , research questions and hypothesis, methods, results and discussion. This section guides the reader through what the problem or research question was, how you conducted the study, how you analysed the data, what you found and what it means.
• Acknowledgements
• Supplementary materials

Strategies to successfully write different types of scientific papers

There are several steps you can take as an academic to prepare yourself for success with publishing any of these types of papers .

• Regularly reading journal articles in your area is invaluable.
• Co-authoring papers with colleagues is another way you can gain experience of the academic writing process without having to travel that journey alone.
• Seeking opportunities to write with more seasoned researchers and benefit from their mentoring is a great way to hone your academic writing skills.
• Another opportunity you should take up is serving as a peer reviewer for a journal and experiencing the review process so you can see how different types of articles are reviewed.

Understanding how different types of papers are written and how they are reviewed will help you decide which type of paper you can publish and add to your CV.

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• A Research Guide
• Research Paper Guide

Different Types of Research Papers

• According to the purpose
• According to the depth of scope
• According to the data type
• According to variables manipulation
• According to the type of inference
• According to the time in which it is carried out
• According to the sources of information
• According to how the data is obtained
• According to design
• Other research paper types

Types of research papers

As a way to make your journey through the research-type paper options a bit easier, let’s divide them by types of work.

According to the purpose:

• Theoretical. Theoretical research type is one of the most popular types of research paper as it has a clear focus. If you have to work with this type, your main objective is to generate all currently available. Even if it has no practical appliance (like in Engineering or design), you must use it anyway. You must collect data and make sure that your target audience understands what your research is about and what theory it follows. Most of such research papers will relate to theories and basic analytical work.
• Applied. This research type stands for something that can be approached scientifically based on practice. The aim here is to generate practical skills. It’s essential in Engineering, Healthcare, and Biology. For such types of papers, one can alternate between technological or scientific types of research, depending on your aims. A technological approach will be fitting if you wish to improve some processes. Now, the scientific research type would include prediction as you work with variables and design things.

According to the depth of scope:

• Exploratory. It is most suitable for research type papers where you have to explore a not-well-known subject. Start with making a hypothesis and developing research. It can be an investigation talking about the role of video games in the development of teenagers.
• Descriptive. This type of research is where you must describe certain characteristics or discuss specifics of some belief or an event. You may not have to research why something has caused these characteristic traits. You must describe and talk about how some things may change IF this or that takes place.
• Explanatory. It’s one of the popular research methods since one has to analyze specific methodologies and help the target audience trace the cause-and-effect relations. It is close to descriptive writing by nature. Still, you must create a research environment since your findings may have to be re-created by others.
• Correlational. This is where you identify the link between two or more variables. You must focus on determining whether certain research variables will be affected and see whether something is systematic regarding these changes (correlational research methodology).

According to the data type:

• Qualitative . It’s used to collect, evaluate, and explain information based on obtained information. It means you have to approach a linguistic-semiotic method to things as you research. You can turn to analysis, interviews, questionnaires, and personal surveys. This is where statistical data helps! You must ask yourself “why” instead of “how.”
• Quantitative. Such types of papers to write belong to one of the most challenging cases because quantitative stands for mathematical (think MATLAB) and computer-based software to check things. It also makes it possible to create a prognosis, which is why this type of research is usually met in engineering.
• Mixed. It’s also possible to use both methodologies if you can support your research type assignment with source information and personal examples. If you are dealing with Psychology or Experimental study, use surveys and aid yourself with AI-based evaluation tools.

According to variables manipulation:

• Experimental. Contrary to its title, you do not have to experiment per se. It’s about the design or replication of things you research. It means you have to re-create specific research conditions to discover what effects are caused by given variables. It’s where you primarily use case studies and sample groups.
• Non-experimental. They often call this research type an observational study. It means that you have to provide analysis in its natural environment. You do not have to intervene in the process but consider turning to descriptive writing. This research may include observation of animals in their natural habitat or the use of the noise effect in the urban environment.
• Quasi-experimental. These types of academic papers are not purely experimental, as you only work with two or three variables. Another aspect of this research is based on randomly chosen variables. It helps to decrease the bias in your study. It also helps to focus on relevant data and allows us to narrow things down.

According to the type of inference:

• Deductive. It means the research is basically fixed since one has to focus on laws and things that can or cannot be. It helps to come to certain conclusions. As you look at the research problem, you use deduction to create your considerations. If you make assumptions and develop reliable evidence, this work method suits you.
• Inductive. It’s one of the flexible methods to think about. The reason why it’s flexible is the way inductive research is generated. You conclude by observing and generalizing while different kinds of research occur. You have to collect data over a period, which makes the process less fixed.
• Hypothetical-deductive approach. You have to make a hypothesis for your research work and use deduction methods to come up with a conclusion. The major difference is that a researcher also takes time to evaluate whether things are correct.

According to the time in which it is carried out:

• Longitudinal. You might know this type of work as diachronic research. Despite the complex name, it focuses on the same issue or an event where a fixed period is taken. It has to track certain changes based on variables. It’s one of the most popular research papers in Healthcare, Nursing, Sociology, Psychology, and Education.
• Cross-sectional. Also known as synchronous research, it is the type of work that approaches cross-sectional design. Here, you have to look at some event or a process at a certain point by taking notes. Thus, research can be used both for sample groups or when working with a case study.

According to the sources of information:

• Primary. Most students are asked to use primary sources. It is exactly why we have a primary research paper method. The data must be collected directly (personal interviews, surveys, questionnaires, a field observation study, etc.) and represent first-hand information. It is perfect for papers in Psychology, Journalism, Healthcare, and subjects where accuracy is vital.
• Secondary. This research type of work is mainly developed with sources that represent secondary references. These include books in print or found online, scientific journals, peer-reviewed documents, etc. If another expert or a student reviews a study, it is related to secondary research; so will your project.

According to how the data is obtained:

• Documentary. As the name suggests, documentary research is based on the secondary references you used. It is a systematic review where you turn to secondary sources related to your subject of study. The most prominent types of research projects in this area are writing a literature review or working with a case study. It is one of the most accessible and clear types of research work.
• Field. It is quite popular research these days as students tend to collect information in the field or at the location where something takes place. Think about researching Fashion Studies where you attend the shows or exploring Environmental Science, where you must observe a phenomenon and take notes.
• Laboratory. The major difference in laboratory research type is working in a strictly-controlled environment where study notes are taken immediately. You must isolate unnecessary variables and use one or two scientific methods. Therefore, such type of research writing is called laboratory research. If your college professor asks for this assignment, consider keeping up with standards and rules.
• Survey. This is where you have to work with the primary information or the use of first-hand data you obtain yourself. It is especially helpful when you work with a group to obtain variables. With this research type, you can also come up with certain conclusions to support your hypothesis and thesis statement.

According to design:

• Fixed. When conducting a fixed research type, narrow things down and focus on temporal aspects. It means you have to discuss how often you will evaluate something, where your research will occur, a sample group, and other fixed variables. Working on fixed types of research reports, creating precise conditions, and follow strict protocols. Such research is related chiefly to lab reports or laboratory works mostly used in Healthcare and/or Law.
• Flexible. Now, the flexible research type will provide you with a process where certain things will change as you take step after step in your research. The examples may include case studies where you have to observe the changes that may take over time. Another example would relate to Anthropology or Geography, where you have to observe a group of people or deal with a cross-cultural analysis. It can also relate to grounded-theory studies, where you should develop theoretical knowledge based on analysis and your thinking.

Other research paper types:

• Argumentative. Also known as a persuasive research type paper, you have to persuade your target audience on your side and a point of view. You have to use at least one piece of evidence (references) to prove your point and support your argument. You must talk about different research opinions and show why your side is correct.
• Analytical. Analytical research papers should always pose a problem and collect relevant information. You can look at another researcher’s works and provide an analysis based on various points of view. The main types of research papers include analysis and must keep the tone analytical and remain neutral without showing your thoughts unless only to guide the reader.
• Definition. This research type requires describing the facts or arguments without using anything based on your opinion or an emotional constituent. You only have to offer information by including facts, yet let your data remain without analysis or bias.
• Action-based. This research type assignment must conduct your work based on a process or a certain action causing things. It can also lead to social processes where a person’s actions have led to something. It can be some research about social movements and/or manufacturing processes.
• Causal. It may relate to cause-and-effect papers where you must focus on the causes. This research type has to address the questions and explore the causes. It can be based on case studies related to business, education, environmental, educational issues, and more.
• Classification. If you have to classify, compare, and contrast things, this method will be helpful. Start with the standards and the rules by setting your classification type immediately. Once you know it, your research paper will go smoothly.
• Comparative. As a rule, this research will deal with comparative work where you take a methodology and compare two sample groups, two individuals, different beliefs, or situations. If you have to compare, discuss your objectives and then create two columns to determine differences and similarities.

What research paper type is most suitable for me?

It will always depend on the research paper objectives you wish to achieve. If you need clarification on the research type you must approach, consult your academic advisor or look closely at your grading rubric. If it says that you must develop an analytical study, it will require posing a specific research question or a problem. The next step would be to collect information on a topic and provide an analysis based on various points of view.

Likewise, if your grading rubric has the word “definition” mentioned, your research type paper must focus on the facts or argumentation. In this case, you should not provide your opinion or talk about what some author thinks. Only the definition of an object or belief is necessary.

As you can see, you only have to find out what your research must achieve. Set the purpose and look at the different types of research and possible methods to approach your problem . Once you know it, look at the research type papers and choose the most fitting option!

• Writing a Research Paper
• Research Paper Title
• Research Paper Sources
• Research Paper Problem Statement
• Research Paper Thesis Statement
• Hypothesis for a Research Paper
• Research Question
• Research Paper Outline
• Research Paper Summary
• Research Paper Prospectus
• Research Paper Proposal
• Research Paper Format
• Research Paper Styles
• AMA Style Research Paper
• MLA Style Research Paper
• Chicago Style Research Paper
• APA Style Research Paper
• Research Paper Structure
• Research Paper Cover Page
• Research Paper Abstract
• Research Paper Introduction
• Research Paper Body Paragraph
• Research Paper Literature Review
• Research Paper Background
• Research Paper Methods Section
• Research Paper Results Section
• Research Paper Discussion Section
• Research Paper Conclusion
• Research Paper Appendix
• Research Paper Bibliography
• APA Reference Page
• Annotated Bibliography
• Bibliography vs Works Cited vs References Page
• Research Paper Types
• What is Qualitative Research

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Scientific and Scholarly Writing

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Parts of a Scientific & Scholarly Paper

Introduction.

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Different sections are needed in different types of scientific papers (lab reports, literature reviews, systematic reviews, methods papers, research papers, etc.). Projects that overlap with the social sciences or humanities may have different requirements. Generally, however, you'll need to include:

INTRODUCTION (Background)

METHODS SECTION (Materials and Methods)

What is a title

Titles have two functions: to identify the main topic or the message of the paper and to attract readers.

The title will be read by many people. Only a few will read the entire paper, therefore all words in the title should be chosen with care. Too short a title is not helpful to the potential reader. Too long a title can sometimes be even less meaningful. Remember a title is not an abstract. Neither is a title a sentence.

What makes a good title?

A good title is accurate, complete, and specific. Imagine searching for your paper in PubMed. What words would you use?

• Use the fewest possible words that describe the contents of the paper.
• Avoid waste words like "Studies on", or "Investigations on".
• Use specific terms rather than general.
• Use the same key terms in the title as the paper.
• Watch your word order and syntax.

The abstract is a miniature version of your paper. It should present the main story and a few essential details of the paper for readers who only look at the abstract and should serve as a clear preview for readers who read your whole paper. They are usually short (250 words or less).

The goal is to communicate:

•  What was done?
•  Why was it done?
•  How was it done?
•  What was found?

A good abstract is specific and selective. Try summarizing each of the sections of your paper in a sentence two. Do the abstract last, so you know exactly what you want to write.

• Use 1 or more well developed paragraphs.
• Use introduction/body/conclusion structure.
• Present purpose, results, conclusions and recommendations in that order.
• Make it understandable to a wide audience.
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Citation Styles Guide | Examples for All Major Styles

Published on June 24, 2022 by Jack Caulfield . Revised on November 7, 2022.

A citation style is a set of guidelines on how to cite sources in your academic writing . You always need a citation whenever you quote , paraphrase , or summarize a source to avoid plagiarism . How you present these citations depends on the style you follow. Scribbr’s citation generator can help!

Different styles are set by different universities, academic associations, and publishers, often published in an official handbook with in-depth instructions and examples.

There are many different citation styles, but they typically use one of three basic approaches: parenthetical citations , numerical citations, or note citations.

Parenthetical citations

• Chicago (Turabian) author-date

CSE name-year

Numerical citations

CSE citation-name or citation-sequence

Note citations

• Chicago (Turabian) notes and bibliography

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Table of contents

Types of citation: parenthetical, note, numerical, which citation style should i use, parenthetical citation styles, numerical citation styles, note citation styles, frequently asked questions about citation styles.

The clearest identifying characteristic of any citation style is how the citations in the text are presented. There are three main approaches:

• Parenthetical citations: You include identifying details of the source in parentheses in the text—usually the author’s last name and the publication date, plus a page number if relevant ( author-date ). Sometimes the publication date is omitted ( author-page ).
• Numerical citations: You include a number in brackets or in superscript, which corresponds to an entry in your numbered reference list.
• Note citations: You include a full citation in a footnote or endnote, which is indicated in the text with a superscript number or symbol.

Citation styles also differ in terms of how you format the reference list or bibliography entries themselves (e.g., capitalization, order of information, use of italics). And many style guides also provide guidance on more general issues like text formatting, punctuation, and numbers.

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In most cases, your university, department, or instructor will tell you which citation style you need to follow in your writing. If you’re not sure, it’s best to consult your institution’s guidelines or ask someone. If you’re submitting to a journal, they will usually require a specific style.

Sometimes, the choice of citation style may be left up to you. In those cases, you can base your decision on which citation styles are commonly used in your field. Try reading other articles from your discipline to see how they cite their sources, or consult the table below.

The American Anthropological Association (AAA) recommends citing your sources using Chicago author-date style . AAA style doesn’t have its own separate rules. This style is used in the field of anthropology.

APA Style is defined by the 7th edition of the Publication Manual of the American Psychological Association . It was designed for use in psychology, but today it’s widely used across various disciplines, especially in the social sciences.

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The citation style of the American Political Science Association (APSA) is used mainly in the field of political science.

The citation style of the American Sociological Association (ASA) is used primarily in the discipline of sociology.

Chicago author-date

Chicago author-date style is one of the two citation styles presented in the Chicago Manual of Style (17th edition). It’s used mainly in the sciences and social sciences.

The citation style of the Council of Science Editors (CSE) is used in various scientific disciplines. It includes multiple options for citing your sources, including the name-year system.

Harvard style is often used in the field of economics. It is also very widely used across disciplines in UK universities. There are various versions of Harvard style defined by different universities—it’s not a style with one definitive style guide.

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MLA style is the official style of the Modern Language Association, defined in the MLA Handbook (9th edition). It’s widely used across various humanities disciplines. Unlike most parenthetical citation styles, it’s author-page rather than author-date.

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The American Chemical Society (ACS) provides guidelines for a citation style using numbers in superscript or italics in the text, corresponding to entries in a numbered reference list at the end. It is used in chemistry.

The American Medical Association ( AMA ) provides guidelines for a numerical citation style using superscript numbers in the text, which correspond to entries in a numbered reference list. It is used in the field of medicine.

CSE style includes multiple options for citing your sources, including the citation-name and citation-sequence systems. Your references are listed alphabetically in the citation-name system; in the citation-sequence system, they appear in the order in which you cited them.

The Institute of Electrical and Electronics Engineers ( IEEE ) provides guidelines for citing your sources with IEEE in-text citations that consist of numbers enclosed in brackets, corresponding to entries in a numbered reference list. This style is used in various engineering and IT disciplines.

The National Library of Medicine (NLM) citation style is defined in Citing Medicine: The NLM Style Guide for Authors, Editors, and Publishers (2nd edition).

Vancouver style is also used in various medical disciplines. As with Harvard style, a lot of institutions and publications have their own versions of Vancouver—it doesn’t have one fixed style guide.

The Bluebook: A Uniform System of Citation is the main style guide for legal citations in the US. It’s widely used in law, and also when legal materials need to be cited in other disciplines.

Chicago notes and bibliography

Chicago notes and bibliography is one of the two citation styles presented in the Chicago Manual of Style (17th edition). It’s used mainly in the humanities.

The Oxford University Standard for the Citation of Legal Authorities ( OSCOLA ) is the main legal citation style in the UK (similar to Bluebook for the US).

There are many different citation styles used across different academic disciplines, but they fall into three basic approaches to citation:

• Parenthetical citations : Including identifying details of the source in parentheses —usually the author’s last name and the publication date, plus a page number if available ( author-date ). The publication date is occasionally omitted ( author-page ).
• Numerical citations: Including a number in brackets or superscript, corresponding to an entry in your numbered reference list.
• Note citations: Including a full citation in a footnote or endnote , which is indicated in the text with a superscript number or symbol.

Check if your university or course guidelines specify which citation style to use. If the choice is left up to you, consider which style is most commonly used in your field.

• APA Style is the most popular citation style, widely used in the social and behavioral sciences.
• MLA style is the second most popular, used mainly in the humanities.
• Chicago notes and bibliography style is also popular in the humanities, especially history.
• Chicago author-date style tends to be used in the sciences.

Other more specialized styles exist for certain fields, such as Bluebook and OSCOLA for law.

The most important thing is to choose one style and use it consistently throughout your text.

A scientific citation style is a system of source citation that is used in scientific disciplines. Some commonly used scientific citation styles are:

• Chicago author-date , CSE , and Harvard , used across various sciences
• ACS , used in chemistry
• AMA , NLM , and Vancouver , used in medicine and related disciplines
• AAA , APA , and ASA , commonly used in the social sciences

APA format is widely used by professionals, researchers, and students in the social and behavioral sciences, including fields like education, psychology, and business.

Be sure to check the guidelines of your university or the journal you want to be published in to double-check which style you should be using.

MLA Style  is the second most used citation style (after APA ). It is mainly used by students and researchers in humanities fields such as literature, languages, and philosophy.

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Caulfield, J. (2022, November 07). Citation Styles Guide | Examples for All Major Styles. Scribbr. Retrieved April 2, 2024, from https://www.scribbr.com/citing-sources/citation-styles/

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Home » Research Methodology – Types, Examples and writing Guide

Research Methodology – Types, Examples and writing Guide

Table of Contents

Research Methodology

Definition:

Research Methodology refers to the systematic and scientific approach used to conduct research, investigate problems, and gather data and information for a specific purpose. It involves the techniques and procedures used to identify, collect , analyze , and interpret data to answer research questions or solve research problems . Moreover, They are philosophical and theoretical frameworks that guide the research process.

Structure of Research Methodology

Research methodology formats can vary depending on the specific requirements of the research project, but the following is a basic example of a structure for a research methodology section:

I. Introduction

• Provide an overview of the research problem and the need for a research methodology section
• Outline the main research questions and objectives

II. Research Design

• Explain the research design chosen and why it is appropriate for the research question(s) and objectives
• Discuss any alternative research designs considered and why they were not chosen
• Describe the research setting and participants (if applicable)

III. Data Collection Methods

• Describe the methods used to collect data (e.g., surveys, interviews, observations)
• Explain how the data collection methods were chosen and why they are appropriate for the research question(s) and objectives
• Detail any procedures or instruments used for data collection

IV. Data Analysis Methods

• Describe the methods used to analyze the data (e.g., statistical analysis, content analysis )
• Explain how the data analysis methods were chosen and why they are appropriate for the research question(s) and objectives
• Detail any procedures or software used for data analysis

V. Ethical Considerations

• Discuss any ethical issues that may arise from the research and how they were addressed
• Explain how informed consent was obtained (if applicable)
• Detail any measures taken to ensure confidentiality and anonymity

VI. Limitations

• Identify any potential limitations of the research methodology and how they may impact the results and conclusions

VII. Conclusion

• Summarize the key aspects of the research methodology section
• Explain how the research methodology addresses the research question(s) and objectives

Research Methodology Types

Types of Research Methodology are as follows:

Quantitative Research Methodology

This is a research methodology that involves the collection and analysis of numerical data using statistical methods. This type of research is often used to study cause-and-effect relationships and to make predictions.

Qualitative Research Methodology

This is a research methodology that involves the collection and analysis of non-numerical data such as words, images, and observations. This type of research is often used to explore complex phenomena, to gain an in-depth understanding of a particular topic, and to generate hypotheses.

Mixed-Methods Research Methodology

This is a research methodology that combines elements of both quantitative and qualitative research. This approach can be particularly useful for studies that aim to explore complex phenomena and to provide a more comprehensive understanding of a particular topic.

Case Study Research Methodology

This is a research methodology that involves in-depth examination of a single case or a small number of cases. Case studies are often used in psychology, sociology, and anthropology to gain a detailed understanding of a particular individual or group.

Action Research Methodology

This is a research methodology that involves a collaborative process between researchers and practitioners to identify and solve real-world problems. Action research is often used in education, healthcare, and social work.

Experimental Research Methodology

This is a research methodology that involves the manipulation of one or more independent variables to observe their effects on a dependent variable. Experimental research is often used to study cause-and-effect relationships and to make predictions.

Survey Research Methodology

This is a research methodology that involves the collection of data from a sample of individuals using questionnaires or interviews. Survey research is often used to study attitudes, opinions, and behaviors.

Grounded Theory Research Methodology

This is a research methodology that involves the development of theories based on the data collected during the research process. Grounded theory is often used in sociology and anthropology to generate theories about social phenomena.

Research Methodology Example

An Example of Research Methodology could be the following:

Research Methodology for Investigating the Effectiveness of Cognitive Behavioral Therapy in Reducing Symptoms of Depression in Adults

Introduction:

The aim of this research is to investigate the effectiveness of cognitive-behavioral therapy (CBT) in reducing symptoms of depression in adults. To achieve this objective, a randomized controlled trial (RCT) will be conducted using a mixed-methods approach.

Research Design:

The study will follow a pre-test and post-test design with two groups: an experimental group receiving CBT and a control group receiving no intervention. The study will also include a qualitative component, in which semi-structured interviews will be conducted with a subset of participants to explore their experiences of receiving CBT.

Participants:

Participants will be recruited from community mental health clinics in the local area. The sample will consist of 100 adults aged 18-65 years old who meet the diagnostic criteria for major depressive disorder. Participants will be randomly assigned to either the experimental group or the control group.

Intervention :

The experimental group will receive 12 weekly sessions of CBT, each lasting 60 minutes. The intervention will be delivered by licensed mental health professionals who have been trained in CBT. The control group will receive no intervention during the study period.

Data Collection:

Quantitative data will be collected through the use of standardized measures such as the Beck Depression Inventory-II (BDI-II) and the Generalized Anxiety Disorder-7 (GAD-7). Data will be collected at baseline, immediately after the intervention, and at a 3-month follow-up. Qualitative data will be collected through semi-structured interviews with a subset of participants from the experimental group. The interviews will be conducted at the end of the intervention period, and will explore participants’ experiences of receiving CBT.

Data Analysis:

Quantitative data will be analyzed using descriptive statistics, t-tests, and mixed-model analyses of variance (ANOVA) to assess the effectiveness of the intervention. Qualitative data will be analyzed using thematic analysis to identify common themes and patterns in participants’ experiences of receiving CBT.

Ethical Considerations:

This study will comply with ethical guidelines for research involving human subjects. Participants will provide informed consent before participating in the study, and their privacy and confidentiality will be protected throughout the study. Any adverse events or reactions will be reported and managed appropriately.

Data Management:

All data collected will be kept confidential and stored securely using password-protected databases. Identifying information will be removed from qualitative data transcripts to ensure participants’ anonymity.

Limitations:

One potential limitation of this study is that it only focuses on one type of psychotherapy, CBT, and may not generalize to other types of therapy or interventions. Another limitation is that the study will only include participants from community mental health clinics, which may not be representative of the general population.

Conclusion:

This research aims to investigate the effectiveness of CBT in reducing symptoms of depression in adults. By using a randomized controlled trial and a mixed-methods approach, the study will provide valuable insights into the mechanisms underlying the relationship between CBT and depression. The results of this study will have important implications for the development of effective treatments for depression in clinical settings.

How to Write Research Methodology

Writing a research methodology involves explaining the methods and techniques you used to conduct research, collect data, and analyze results. It’s an essential section of any research paper or thesis, as it helps readers understand the validity and reliability of your findings. Here are the steps to write a research methodology:

• Start by explaining your research question: Begin the methodology section by restating your research question and explaining why it’s important. This helps readers understand the purpose of your research and the rationale behind your methods.
• Describe your research design: Explain the overall approach you used to conduct research. This could be a qualitative or quantitative research design, experimental or non-experimental, case study or survey, etc. Discuss the advantages and limitations of the chosen design.
• Discuss your sample: Describe the participants or subjects you included in your study. Include details such as their demographics, sampling method, sample size, and any exclusion criteria used.
• Describe your data collection methods : Explain how you collected data from your participants. This could include surveys, interviews, observations, questionnaires, or experiments. Include details on how you obtained informed consent, how you administered the tools, and how you minimized the risk of bias.
• Explain your data analysis techniques: Describe the methods you used to analyze the data you collected. This could include statistical analysis, content analysis, thematic analysis, or discourse analysis. Explain how you dealt with missing data, outliers, and any other issues that arose during the analysis.
• Discuss the validity and reliability of your research : Explain how you ensured the validity and reliability of your study. This could include measures such as triangulation, member checking, peer review, or inter-coder reliability.
• Acknowledge any limitations of your research: Discuss any limitations of your study, including any potential threats to validity or generalizability. This helps readers understand the scope of your findings and how they might apply to other contexts.
• Provide a summary: End the methodology section by summarizing the methods and techniques you used to conduct your research. This provides a clear overview of your research methodology and helps readers understand the process you followed to arrive at your findings.

When to Write Research Methodology

Research methodology is typically written after the research proposal has been approved and before the actual research is conducted. It should be written prior to data collection and analysis, as it provides a clear roadmap for the research project.

The research methodology is an important section of any research paper or thesis, as it describes the methods and procedures that will be used to conduct the research. It should include details about the research design, data collection methods, data analysis techniques, and any ethical considerations.

The methodology should be written in a clear and concise manner, and it should be based on established research practices and standards. It is important to provide enough detail so that the reader can understand how the research was conducted and evaluate the validity of the results.

Applications of Research Methodology

Here are some of the applications of research methodology:

• To identify the research problem: Research methodology is used to identify the research problem, which is the first step in conducting any research.
• To design the research: Research methodology helps in designing the research by selecting the appropriate research method, research design, and sampling technique.
• To collect data: Research methodology provides a systematic approach to collect data from primary and secondary sources.
• To analyze data: Research methodology helps in analyzing the collected data using various statistical and non-statistical techniques.
• To test hypotheses: Research methodology provides a framework for testing hypotheses and drawing conclusions based on the analysis of data.
• To generalize findings: Research methodology helps in generalizing the findings of the research to the target population.
• To develop theories : Research methodology is used to develop new theories and modify existing theories based on the findings of the research.
• To evaluate programs and policies : Research methodology is used to evaluate the effectiveness of programs and policies by collecting data and analyzing it.
• To improve decision-making: Research methodology helps in making informed decisions by providing reliable and valid data.

Purpose of Research Methodology

Research methodology serves several important purposes, including:

• To guide the research process: Research methodology provides a systematic framework for conducting research. It helps researchers to plan their research, define their research questions, and select appropriate methods and techniques for collecting and analyzing data.
• To ensure research quality: Research methodology helps researchers to ensure that their research is rigorous, reliable, and valid. It provides guidelines for minimizing bias and error in data collection and analysis, and for ensuring that research findings are accurate and trustworthy.
• To replicate research: Research methodology provides a clear and detailed account of the research process, making it possible for other researchers to replicate the study and verify its findings.
• To advance knowledge: Research methodology enables researchers to generate new knowledge and to contribute to the body of knowledge in their field. It provides a means for testing hypotheses, exploring new ideas, and discovering new insights.
• To inform decision-making: Research methodology provides evidence-based information that can inform policy and decision-making in a variety of fields, including medicine, public health, education, and business.

Advantages of Research Methodology

Research methodology has several advantages that make it a valuable tool for conducting research in various fields. Here are some of the key advantages of research methodology:

• Systematic and structured approach : Research methodology provides a systematic and structured approach to conducting research, which ensures that the research is conducted in a rigorous and comprehensive manner.
• Objectivity : Research methodology aims to ensure objectivity in the research process, which means that the research findings are based on evidence and not influenced by personal bias or subjective opinions.
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Review of research and control technology of underwater bionic robots

• Open access
• Published: 18 October 2023
• Volume 1 , article number  7 , ( 2023 )

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• Zhongao Cui 1   na1 ,
• Liao Li 1   na1 ,
• Yuhang Wang 1 ,
• Zhiwei Zhong 1 &
• Junyang Li 1  

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As marine resources continue to be exploited, the remarkable locomotion and coordination of fish provide an excellent source of inspiration for scientists and engineers to design and control the next -generation autonomous underwater vehicles within a bionic framework. Underwater biomimetic robots combine bionics and robot technology, and their biological characteristics offer a lot of convenience for the robot so that it can obtain better performance in adaptability and robustness. Recently, with the combination of bionics, mechanics, electronics, materials science, and automation, there has been great progress in developing underwater bionic robots with different structure types and energy supply modes. This paper summarizes the research status of underwater robots, focuses on the research status of underwater bionic robots with different materials, types and motion modes, and introduces the propulsion mechanism of underwater robots with different structures and the control methods adopted in the propulsion process. Finally, the broad application prospect and market potential of underwater biomimetic robot are introduced.

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1 Introduction

A bionic underwater robot, as the name suggests, is a new type of robot that imitates the propulsion mechanism and body structure of fish or other marine creatures living underwater using electromechanical components and intelligent materials (such as memory alloy materials, mixed materials, and rigid materials), which can adapt to different underwater environments and realize underwater propulsion (Chu et al. 2012 ). It has the characteristics of high efficiency, high mobility, and low noise (Chen et al. 2021a ). For a long time, scholars have been committed to studying marine biological propulsion models and bionic underwater robots. Underwater vehicles can be classified into two groups based on their structural design: cabled underwater vehicles, commonly known as remotely operated vehicles (ROVs), and cableless underwater robots, traditionally known as autonomous underwater vehicles (AUVs) (Wynn et al. 2014 ). Moreover, they can be categorized by use into underwater investigation robots (observation, measurement, test material collection, etc.) and underwater operation robots (underwater welding, pipe twisting, underwater construction, underwater cutting, etc.) (Vu et al. 2018 ).

At present, most underwater robots are frame-based, similar to the rotating elongated body of a submarine. With the continuous development of bionic technology, the bionic fish shape and control modes of underwater robots will also evolve (Xie et al. 2021 ). In this review, different control system algorithms are described, such as those developed for individual or cluster control of underwater robots (Khalaji and Zahedifar  2020 ). Underwater robots work in unknown and challenging marine environments. Complex marine environments, such as wind, waves, currents, and water pressure, severely interfere with robot motion and control, making communication and navigation of underwater robots very difficult. Thus, the development potential of underwater robots still needs to be continuously explored. In this paper, various underwater robots are reviewed and introduced from manufacturing materials, structural design, drive mechanism, and control strategy.

1.1 Shape memory alloy material

Shape memory alloys (SMAs) are solid, smart materials driven by current silently. The principle of operation is that when heated from low-temperature martensite to high-temperature austenite, SMA returns to the predetermined shape and generates activation, a process known as the reversed-phase transition (Yang et al. 2023 ). When cooled from austenite to martensite, SMA experiences a martensitic phase transition and returns to its initial state under bias stress (Hu et al. 2023 ). Previous works reported that the bionic starfish robot and the bionic manta ray robot fish were both powered by SMA.

The jellyfish robot, or Robojelly, developed by Virginia Tech, is driven by a bionic shape memory alloy composite actuator modeled after a jellyfish. With a body made of RTV silicone with a total mass of 242 g and a bell-shaped diameter of 164 mm, Robojelly can generate enough thrust to propel itself in static water conditions (Villanueva et al. 2011 ) (Fig.  1 a). Harbin Engineering University developed an underwater jellyfish microrobot prototype model based on SMA and ionic conductive polymer film (ICPF) as actuators to achieve swimming movement, with an overall size of microrobot of about 75 mm long, 55 mm in diameter, and 6.5 g weight. This tiny jellyfish-like robot has four tentacles (Fig.  1 b). Each mechanism consists of a restraint mechanism and an ICPF actuator, and each tentacle can work with an SMA driver to increase its range of motion and provide greater propulsion. The energized SMA shrinks the internal volume of the microrobot so that the water or other water-containing medium inside the microrobot is driven backward, thus forming a propulsion force (Yang et al. 2007 ). Another jet-propelled jellyfish bionic robot, MPA-O, developed by Harbin Engineering University, is made of SMA material. The length of the moving direction is 46.1 mm, and the section diameter is 36.3 mm. At an operating frequency of 0.6 Hz, the robot has a maximum speed of 6 mm/s (Guo et al. 2007 ) (Fig.  1 c).

SMA material underwater bionic robot. a Jellyfish robot (Villanueva et al. 2011 ); b  Jellyfish-like microrobot (Yang et al. 2007 ); c  Jellyfish-like micro robot that achieves MPA-O swimming mode using a hybrid actuator (Guo et al. 2007 ); d  Structure of the jellyfish-like robot (Ko et al. 2012 ); e  Microrobot manta ray (Wang et al. 2009 ); f  Fish skeleton structure including a latex-based skin for water protection (Rossi et al. 2011 )

The miniature jellyfish swimming robot, powered by the SMA system developed by Chonnam National University, has four flexible fins, each equipped with a permanent magnet for electromagnetic drive, and the body of the robot has a length of 17 mm and a thickness of 0.5 mm. The SMA driver can generate a uniform magnetic field in the desired direction in 3D space, which can bend the fins of the jellyfish-like microrobot. Thus, the cyclic changes in the uniform magnetic field will synchronize the fluctuations of the fins and generate a propulsive force for the robot in the desired direction (Ko et al. 2012 ) (Fig.  1 d). A miniature bionic manta ray robotic fish with a triangular pectoral fin driven by SMA, developed by the Harbin Institute of Technology, is based on the simplified pectoral fin model described in the joint. Each consists of a bionic fin on the leading edge and a latex film (0.2 mm thick) that forms the surface of the fin. The front part of the tail is attached to the body and is a flexible fin that can adjust the course (Wang et al. 2009 ) (Fig.  1 e). The SMA is used as a continuous backbone for curved fish, and the University of Madrid utilized six SMA-based actuators to make the skeleton of the robotic fish. Their length is 1/3 of their body length (8.5 cm, excluding the tail fins and head). They are positioned in pairs parallel to the body so that antagonistic motion can enable the robotic fish to resist higher water pressure (Rossi et al. 2011 ) (Fig.  1 f).

Jellyfish robots developed at Kagawa University achieved flutter-like motion with SMA-based actuators and positive spring elements (Najem et al. 2012 ). The robot was 63 mm long, 35 mm wide, and 18 mm high, the same size as the manta ray robot previously developed by the laboratory, which was constructed with SMA wires embedded in an elastic substrate (Xie et al. 2018 ). The jellyfish robot’s fluctuating motion is generated by 10 SMA actuators (5 on each side) with a maximum speed of 40 mm/s at 3.125 Hz, and the Department of Health Sciences and Technology of ETH Zurich (Vogel 2012 ) developed another jellyfish-like robot with jet propulsion via an SMA-based actuator. The drive is called bionic shape memory alloy composite (BISMAC). BISMAC is assembled from a steel spring and SMA wire embedded in a silicone precision connection (Ma et al. 2019 ) and has a length of 110 mm and thickness of 0.1 mm, while the bell has diameters of 134 and 82 mm. The robot is 242 mm long, 225 mm wide, and 52 mm high. Because its main component is silicone, it has greater hardness, strong compressive performance, and a speed of 35 mm/s. Traditional robots are machined from rigid materials, which often limits their ability to deform and adapt their shape to the external environment. Although these rigid robots have the advantages of large output force, high precision, and strong controllability, they often lack the multifunctional characteristics of natural organisms. Flexible robots comprising SMA materials can achieve elastic deformation and pass through narrow spaces without causing internal damage (Hu et al. 2019 ).

1.2 Ionic polymer–metal composite

Biomimetic artificial muscle material is a new type of intelligent material developed rapidly in the 1990s, constantly setting off a global research upsurge and has important application value in the aerospace, biomimetic robot, and biomedical engineering fields. Ionic polymer-metal composites (IPMC), as electrochemical actuators, are typical biomimetic artificial muscle materials (Şafak and Adams 2002 ) with a sandwich structure comprising two layers of electrodes and ionic polymers. Under the electric field, electrical and mechanical energy can be converted by the reversible dissociation process of ions at the electrode interface. IPMC material has the advantages of fast response, large drive displacement, and low drive voltage. However, it can only be used in wet environments and has huge limited applications in amphibious bionic robots (Cao et al. 2022 ).

IPMC is widely used in the manufacture of body or caudal-fin (BDF) swimming robots, and the fish-like robot developed by the New York Institute of Technology is designed with IPMC materials, which mimics the general swimming movement of a fish, protecting itself with its tail fin (Marras and Porfiri 2012 ). In 2010, Michigan State University developed a wireless bionic robotic fish that also successfully demonstrated the swimming mode of the BCF using a robot based on IPMC as the driving material, which had four different types of fins mounted on its tail to optimize the relationship between robot speed and fin shape (Brown and Clark 2010 ). The University of Science and Technology of China employed the IPMC brake as the tail fin of the robot fish for propulsion, which mainly comprises two servo motors, namely, angle rotating block pairs and brakes, and the main body is arranged symmetrically. The experiment confirms that the robot fish with the two degrees of freedom caudal-fin propulsion mechanism can realize various basic swimming movements by using a caudal fin (Zhou et al. 2017 ) (Fig.  2 a). The University of Nevada studied a biomimetic jellyfish robot powered by an IPMC placed inside a silicone dome. Because the selected jellyfish shell elastomer material is soft enough, the IMP can be easily driven without hindrance, and the robot can increase thrust production by approximately 1300% compared to a normal jellyfish (Trabia et al. 2016 ) (Fig.  2 b).

IPMC material robot fish. a Prototype of the robotic fish (Zhou et al. 2017 ); b  Robot jellyfish prototype (Trabia et al. 2016 ); c  IPMC material jellyfish robot (Yeom and Oh 2008 ); d  Robot fish propelled by IPMC (Hu and Zhou 2009 )

The University of Virginia developed a biomimetic robot that mimics a manta ray’s pectoral fin, which partially comprises a PDMS (polydimethylsiloxane) membrane using four IPMCs (0.28 mm thick) (Sankaranarayanan et al. 2008 ). Jeonnam National University utilized IPMC actuators to build and evaluate biomimetic jellyfish robots. The existing IPMC actuators limit their application fields due to their flat shape, which is a severe defect of this actuator material. To overcome the disadvantages of planar IPMC actuators, a curved IPMC actuator with predetermined initial deformation is developed. The expected initial deformation is acquired by heat treatment. The bionic input signal is generated by imitating the real movement of jellyfish (Yeom and Oh 2008 ) (Fig.  2 c). The experiment confirms that the jellyfish robot can move normally. IPMC is an important electroactive polymer (artificial muscle) with built-in drive and sensing capabilities. The robotic fish developed by the Intelligent Microsystems Laboratory at Michigan State University comprises IMPC material. Its motion mechanism includes the following: IPMC usually includes a thin ion exchange membrane, which is chemically plated on two surfaces with precious metals as electrodes. Application of voltage to the IPMC leads to the transport of hydrated cations and water molecules within the membrane and the associated electrostatic interactions to result in bending motion, leading to a driving effect (Hu and Zhou 2009 ) (Fig.  2 d). The artificial muscles of biomimetic robots’ balance drive the performance, power-to-weight ratio, and muscle form factors. As such, they are ideally suited as biomimetic actuators for various robotic applications. In the past decade, research and application of robotic artificial muscles have been developed (Wynn et al. 2014 ). More fundamental research is required regarding how artificial muscles can be manufactured, modeled, controlled, and engineered to acquire fish-like muscle properties and achieve muscle-like behavior.

1.3 Piezoelectric composite ceramics

A group of researchers from the Artificial Muscle Research Center at Konkuk University in South Korea developed a bionic fish robot that utilizes its tail fin to drive the swimming motion of BCF (Pham et al. 2023 ) and used a lightweight piezoelectric composite ceramic (PZT), a single crystal piezoelectric ceramic encased in glass/epoxy and carbon/epoxy resins, as the actuator material. The robot uses a crank, rack, and pinion structure (the size of the robot is increased by 400 mm due to the need for an additional device to achieve the movement). The robot has a maximum speed of 25.16 mm/s at an operating voltage of 300 vpp and an operating frequency of 0.9 Hz.

The miniature underwater mobile robot developed by Professor Toshio Fukuda of Nagoya University in Japan contains piezoelectric ceramics to drive the oscillation of two symmetrical legs to realize its movement. The two legs of the robot are equipped with a pair of symmetrical fins at a certain angle. The symmetrical structural design can offset the lateral force and strengthen the forward momentum. A 250-fold elastic hinge amplification mechanism is designed to amplify the PZT. The robot is 320 mm long and 190 mm wide, and the motion speed is 21.6–32.5 m/s. In 2009, the Marine Science Center of Northeastern University in the United States developed a robot fish with wave propulsion using PZT materials and chain rod structure (Zhou et al. 2008 ). Through lateral body fluctuations, the robotic eel drives itself through the water column and controls its floating depth. In 2008, DRAPER Lab launched VCUUV (Vorticity Control Unmanned Underwater Vehicle), a piezoelectric ceramic-driven robot fish designed after tuna (Suk and Hwan 2014 ). It is about 2.4 m long and weighs 300 lbs. Its maximum swing frequency is 1.5 Hz, with a maximum swimming speed of 1.25 m/s at 1 Hz. The goal of the laboratory is to develop an autonomous underwater vehicle using eddy current-controlled propulsion and show that PZT materials have good drag reduction characteristics, excellent maneuverability, depth-holding ability, and higher acceleration and deceleration ability through free swimming. Harbin Engineering University (Yue et al. 2015 ) developed a water microrobot with a PZT drive; the main feature of this drive is that it is a polymer material actuator only in water or wet environment work. The robot, which can be turned forward, left, or right, has a pair of driving wings driven by a pulse voltage to generate propulsion. PZT material can realize the continuous fluctuation deformation of the fluctuating fins of bionic fish, which makes it compact in structure, light in weight, and high in efficiency. This kind of robot has broad application prospects and value in microtubule detection and biomedicine.

1.4 Mixed materials

The underwater environment is complex, so the material requirements of the underwater bionic robot are very strict. Currently, polymer-metal composite materials are widely employed, combining the advantages of both the polymer and the metal. Polymers can withstand a certain degree of deformation in most environments. Both materials can make good adjustments to the impact of the external environment, with the polymer being lighter and the metal material being harder (Zheng et al. 2020 ). The robot fish developed by Shandong University of Technology is made of a resin-mixed material and a rigid motor, with four main parts: two laminated tail fins, a rigid fish body with a permanent magnet at the tail, a miniature battery, and a controller. During the driving process, electrical energy is converted into mechanical energy of the tail fin, producing the swimming motion of the robotic fish (Yan et al. 2021 ) (Fig.  3 a). Kagawa University has developed a medusa-like underwater bionic microrobot based on SMA and artificial muscles. It moves like a jellyfish, floats and sinks, and has two pectoral fins to achieve swimming motion (Shi et al. 2010 ) (Fig.  3 b). The mollusk developed by Zhejiang University includes a steering electronic server, a steering tail, and two SMA flapping wings. Two dielectric elastomer (DE) membranes are clamped onto the electrodes to form an artificial muscle. Precut frames and precut rebar are glued to each side of the muscle. The purpose of the insulation board is to prevent the feed pipe to make contact with the support frame. The flexible wavy fins provide power when the wings are in the flapping, stretching, or actuating state. When AC voltage is applied, the wing changes back and forth between the former state and the driven state, providing forward force (Zhang et al. 2021 ) (Fig.  3 c).

Mixed-material robot. a Composite robotic fish structure (Yan et al. 2021 ); b  Prototype jellyfish-like biomimetic underwater microrobot (Shi et al. 2010 ); c  Mechanism composition of the soft robotic fish (Zhang et al. 2021 ); d  Dolphin robot (Shen et al. 2013 ); e  Composite robotic fish (Xie et al. 2020 ); f  Illustration of the robotic fish (Marras and Porfiri 2012 )

DE, which is widely used in robot drives, has good softness, and its outstanding advantages are that the relative adjustment rate after shape change is fast, the response is quite rapid, the energy consumption is less, and the mechanical and electrical conversion efficiencies are high. The dielectric elastic material-driven robot developed by Kagawa University is a jet propulsion robot simulating a pike (Bal et al. 2019 ). The driver is composed of SMA, ICPF, and rubber materials. The length of the motion direction is 46.1 mm, the diameter of the section is 36.3 mm, and the maximum speed of the robot is 6 mm/s. The dolphin robot developed by Beihang University consists of three parts: (1) a rigid plastic shell that acts as a body, (2) IPMC stripes that act as muscles, and (3) a plastic sheet that mimics a tail fin. The shell is designed based on the proportions of the dolphin’s streamlined body, made of nylon plastic, using a 3D printer, and covered with a black matte resin varnish, leading to a smooth surface. The IPMC is attached to the body by two small rectangular conductive copper plates, which act as clamps, with a flexible fin attached to the end of the IPMC, which is designed based on the shape of a natural dolphin fin. The robot can jump and swim freely like a dolphin (Shen et al. 2013 ) (Fig.  3 d). The bionic robotic fish developed by the Chinese University of Hong Kong includes a rigid head, a wired-driven active body, and a flexible tail. A pair of SMA spring plates with the same stiffness pass through an active body comprising multiple connecting rods, which are like the backbone of a real fish, and then distribute a pair of wires along the spring plate to drive the moving body. The robotic fish tail is a flexible tail made of silicone and carbon fiber reinforced material that allows the robotic fish to swim in multiple modes, such as cruising, turning, rising, and descending (Xie et al. 2020 ) (Fig.  3 e). The robotic fish designed by the New York University consists of a rigid acrylonitrile butadiene styrene (ABS) plastic body shell and a tail consisting of rigid ABS elements and flexible polyester tail fins. The robot fish uses a waterproof servo motor to control the tail, and a flexible tail fin allows the tail to bend and undulate to mimic the swimming of a live fish. The tail beat frequency and amplitude of the robot are controlled by an external microcontroller. The signals driven by the servo motor generate the periodic sinusoidal movement of the flexible polyester tail fin to mimic the movement of fish (Marras and Porfiri 2012 ) (Fig.  3 f).

2 Underwater robot control system classification

At present, the commonly used motion control methods of underwater biomimetic robots are model-based control methods, sine controllers, and central mode generator (CPG)-based methods. As the structural components of marine biomimetic robots usually include power modules, sensors, chips, and driving components, the behavior of the bionic robot is controlled by the predefined program or the command controller (the power supply of the controller is mainly provided by traditional lithium batteries) (Chen et al. 2021b ). Depth adjustment of the robot in water is controlled by the controller and is mainly completed by the buoyancy unit. Thus, control can also be divided into rigid motor control and soft drive control.

2.1 Model control method

The model control method is performed by analyzing the dynamics and kinematics of the robot and then establishing a complex mathematical model. The mathematical model can accurately calculate the next movement of the robot to achieve the effect of precise control. However, due to the complex and changeable underwater environment, accurately modeling the robot is very difficult. Even if it can be accurately modeled, its control mode is very complex. In 2014, Inner Mongolia University of Technology (Li et al. 2014b ) developed a set of integrated and efficient driving devices that can control the swing of the fishtail to achieve different amplitudes, different frequencies, and different central positions and realize the functions of acceleration, deceleration, and steering of the released robot fish. Based on elastic plate deformation theory, the design size and motion input of the elastic plate are inversely solved according to the motion function of the actual fish, which makes the deformation motion of the elastic plate highly fit the fishtail swing in reality. The 3D modeling and fluid simulation of the fish body were performed, and the geometric size and motion mode of the prototype were optimized. The bionic robotic fish has good sealing properties in water and can adjust its posture to achieve the flipping and pitching functions. In 2019, the School of Mechanical Engineering, Baicheng Normal University (Wang et al. 2019a ) proposed the concept of 'fundamental wave', including deformation description and linear density description, established the fish body wave model of the bionic robotic fish, formed the control method system of the multijoint bionic robotic fish’s stable swimming propulsion, and achieved the efficient and stable swimming of the bionic robotic fish. In 2022, an underwater soft robot was successfully developed by a joint team from the Max Planck Institute for Intelligent Systems in Germany, Seoul National University in South Korea, and Harvard University in the United States (Ning et al. 2022 ). The robot can swim underwater like a fish and automatically adjust its swing in the water according to the speed of the water. To design the controller for the robotic fish, the research team developed a data-driven, lumped parameter modeling method, which allows for accurate but lightweight simulations using experimental data and genetic algorithms, and the model can accurately predict the robotic fish’s behavior at drive frequency and pressure amplitude, including the effects of antagonistic co-contraction on soft actuators (Li et al. 2023 ). Currently, most of the simplified mathematical models are used for control. Still, the accuracy of the simplified mathematical models is poor, and the robustness of the control system is poor, which makes the underwater bionic robots designed by this method have poor adaptability to the underwater environment.

2.2 Central pattern generator (CPG)

The main control principle of the CPG is to utilize the mathematical model of the neuron network to drive the joint movement by imitating the movement law and biological control mechanism of the animal itself. The School of Intelligent Systems Science and Engineering at Harbin Engineering University (Wang et al.  2019b ) used four oscillators to construct a CPG network model to control the pectoral fin and tail fin with two degrees of freedom of multimode bionic robotic fish, which introduces the angle between the head and tail axis and the horizontal plane and the yaw angle as feedback information to control the swimming posture of the robotic fish and conducted in-depth discussion on the motion control of the pectoral fin. The basic swimming strategy is developed based on Walker’s oscillating pectoral fin model. Based on the multijoint robot fish model, the National University of Singapore extracted two basic imitation swimming modes, 'cruise' and 'C-type sharp turn', from the swimming observation of real fish as training samples. The general internal model imitates the CPG of the nervous system used to learn and regenerate the coordinated behavior of fish. This learning method can use general function approximation capability and time/space scalability to generate the same or similar fish swimming patterns by adjusting two parameters. The learned swimming mode was realized in the experiment of multiarticular robotic fish (Ren et al. 2013 ) (Fig.  4 a). Waseda University built and studied a CPG network with nonlinear oscillators for the gait generation of robotic fish and developed a robot that uses a CPG for fish-like motion underwater. These studies reveal that CPG-based approaches are easy to design, fast to implement, and capable of online adjustments (Chen et al. 2020 ) (Fig.  4 b).

CPG controls the bionic robot. a Robotic fish covered with waterproof tape swimming in the water (Ren et al. 2013 ); b  Fabrication process of the silicone tail and the outer view of the robotic fish (Chen et al. 2020 ); c  Closed-loop CPG-based control can drive the robot fish (Chen et al. 2021b ); d  Prototype of multimode robotic fish (Zhao et al. 2006 )

The CPG model includes four input parameters, namely, flutter amplitude, flutter angular velocity, flutter offset, and the time ratio of the beat phase to the recovery phase in the flutter. The robot fish developed by the South China University of Technology is equipped with three infrared sensors installed on the left, front, and right sides of the robot fish, as well as an inertial measurement unit that can sense the surrounding obstacles and the direction of movement. Based on these sensor signals, CPG-based closed-loop control can drive the robotic fish to avoid obstacles and track the specified direction (Chen et al. 2021b ) (Fig.  4 c). The Peking University-developed robotic fish uses CPG modeling as a nonlinear oscillator for joints to realize coordination by altering the connection weights between joints. The online gait generation method based on CPG makes the transition between swimming gaits elegant and smooth to realize multimode swimming and achieve a more realistic movement. By changing the CPG parameters, various swimming patterns can be obtained to simulate the various movements of real fish in nature or designed based on special tasks (Zhao et al. 2006 ) (Fig.  4 d). The Chinese Academy of Sciences (Yu et al. 2016 ) proposed a particle swarm optimization (PSO)-based CPG control system for underwater vehicles. In general, the parameters of the CPG are determined manually based on experience and computer numerical simulation. In this method, the traveling wave parameters of robotic fish are given manually, and 19 parameters, such as the optimal CPG connection weight, self-inhibition coefficient, and time constant, are selected through the PSO algorithm according to the fish body wave equation. Simulation and experiment show the effectiveness of this method. The Hirose Laboratory of Tokyo Institute of Technology (Nagai and Shintake  2022 ) adopted the CPG control network comprising this oscillator to control the robot, that is, a multijoint snake robot. The robot has 10 actuating units, constituting a bilateral wave propulsion mechanism with bionic left and right counter muscles. The CPG control network can generate rhythm joint angle control signals and achieve the yaw maneuvering of the robot. The simulation test confirms the feasibility and effectiveness of the control system (Alexander  2017 ). This control method simulates the central nervous system well, generates continuous and coordinated control signals, and then gives timely feedback to different environments. This method is conducive to coordinated control and has a better environmental adaptation effect, so it is widely used.

2.3 Sinusoidal controller control method

A sine controller is a kind of control method that is widely used by researchers based on the fact that the waveforms and motion periods generated by how fish are propelled are similar to sine functions. Thus, the sine controller simplifies the motion process of fish into the frequency, amplitude, and waveform of the sine function and then controls the motion of each joint of the underwater bionic robot through these parameters. At the same time, it changes the motion state by relying on the phase difference of the motion between different joints of the robot. The advantages of this control mode are simplicity and easy controllability. In 2015, the robot fish 'Pike' was born at the Massachusetts Institute of Technology; the hardware system of the robot fish 'Pike' includes a head, a pectoral fin, a tail fin, a dorsal fin, a main servo sine controller system, a pectoral fin servo system, and a battery (Li 2015 ). In 2014, the Tokyo Institute of Technology developed a self-propelled robot dolphin with two joints and an autonomous drive controller (Nakashima and Karako 2014 ). The robot dolphin is a simplified model of a high-speed swimming marine creature with a length of 1.7 m, which is very close to the size of the actual dolphin. The robot dolphin has a linear body and a rectangular tail fin. An air motor drives the first joint, and the second is driven by a spring. A measurement system is developed to measure the torque and angle of the first joint. The Polytechnic University of Milan (Bottasso et al. 2008 ) successfully controlled a pair of pectoral fin joints and caudal-fin joints of a robotic fish by using a sinusoidal controller and vibrator (a topology with three oscillators adjacent to each other). By movement of pectoral and caudal joints, the robot can achieve various underwater swimming actions. The experimental results show that the control method can realize stable swimming. Due to the uniqueness of the function types in the controller, this method has limitations. If there is a motion mode that does not belong to the function characteristics, it cannot be accurately regulated. In addition, this control mode has poor adaptability when dealing with the sudden change of control parameters, and it cannot quickly adjust from one motion mode to another, leading to poor environmental adaptability of the robot.

2.4 Rigid motor drive

Most marine bionic robots are driven by rigid motors. Since motor-driven robots are easier to implement in terms of systems than flexible-driven robots, which can fully use the high energy density and high efficiency of motors, rigid motor-driven bionic robots are more convenient for specific purposes (Karthik 2014 ). They are currently more mature in development than flexible-driven bionic robots. For rigid motors, waterproof housing is often needed, with high sealing requirements and greater challenges in terms of water pressure (Dawson and Allison 2020 ).

For bionic robots driven by rigid motors, there are mainly single-motor drives and multimotor drives. Multimotor drive means that the system has more flexibility, but there are more limitations regarding structure and size, and it can carry many functional sensors. Examples include the UK Natural Environment Council’s (NERC) 5.5-m long 1800 kg-dry weight Autosub6000 AUV, which is rated to a depth of 6000 m, can be equipped with a variety of payloads for marine geoscience research, includes high-resolution multibeam echo sounders, seabed profilers, and side-scan sonar, color camera systems, conductivity, temperature, depth, and electrochemical redox (Eh) sensors. It has precise navigation and terrain tracking capabilities and has a sophisticated collision avoidance system (Wynn et al. 2014 ) (Fig.  5 a).

Rigid motor-driven robot. a AUV submarine (Wynn et al. 2014 ); b  SPC-I (Wang et al. 2005 ); c  SPC-II (Liang et al. 2011 ); d  Working environment of SPC-III in the Taihu Lake (Liang et al. 2011 ); e  G9 robotic fish profile (Liu and Hu 2006 ); f  Top view of the robotic fish representing the robot’s undulating tail (Kopman and Porfiri 2013 )

The Robotics Institute of Beihang University successfully developed the bionic robotic eel, bionic robotic dolphin, experimental small robotic fish, and trail-tail bionic robotic fish SPC-I, SPC-II, and SPC-III (Wang et al. 2005 ) (Fig.  5 b) driven by an electric motor and wireless remote-control rigid actuator (Li and Jiang 2012 ), as shown in Figs.  5 c and d. Compared with conventional motor-driven robots, the maneuverability of the bionic underwater robot is significantly enhanced. This series of robots were applied to underwater archaeological discovery, experimental teaching, ocean cruise experiments, and water quality detection and achieved good results. The Harbin Engineering University-developed bionic underwater robot is driven by two servo rigid motors with tail fins and an interactive gear system, which can achieve various complex movements, as well as two articulated serpentines, HRF-I and HRF-II bionic robotic fish (Tian et al. 2022a ). Compared with the former, the latter can achieve steering, snorkeling, and reversing, and the performance in all aspects has been greatly improved. The Department of Computer Science at the University of Essex conducted experiments with a rigidly driven robotic fish G9 equipped with a variety of sensors and found that it can respond to dynamic changes in its environment, capturing its position in the tank and the robot’s posture and internal state, with good drive performance (Liu and Hu 2006 ) (Fig.  5 e). The New York University Institute of Technology designed a robot’s body shell comprising a payload and a motor bay. The payload bay contains control electronics, batteries, and counterweights to enhance pitch and roll stability and achieve appropriate buoyancy. More specifically, buoyancy is set so the robot is almost completely submerged during operation. The cap provides a waterproof seal for the payload bay and extends toward the rear of the robot, partially covering the engine room. A toggle switch hidden in the lid extension turns the robot on or off. The motor compartment houses a Traxxas 2065 waterproof servo motor for the drive, which is connected to the rear by an improved servo motor horn. The caudal fin is snug in a slit at the free end of the caudal fin (Kopman and Porfiri 2013 ) (Fig.  5 f).

Single motor-driven marine bionic robots are often used in fish bionic robots, which have a single function and are not flexible enough in movement, such as the bionic fish studied by Northern Research Center for Science and Technology at Malek Ashtar University of Technology (Sabet and Nourmohammadi 2022 ) and the voice-activated soft robot fish studied by Robert. The Massachusetts Institute of Technology (MIT) Distributed Robotics Laboratory developed a single-motor driven robotic fish, a soft robotic fish system whose subcomponents include an elastomeric tail, an external gear pump, two diving surfaces, and control electronics, including an acoustic receiver and a fish eye-eye camera that can complete underwater reconnaissance missions (Katzschmann et al. 2018 ) (Fig.  6 a). The robotic fish designed by the College of Worcester employs a flexible body with embedded rigid actuators that mimic the elongated anatomical form of a fish. Also, the robot has a novel fluid drive system that drives body movement and has all the subsystems of traditional robots: power, drive, handling, and control. A set of fluid elastomer actuators is at the heart of the fish’s soft body. The soft robot has an input-output relationship similar to a biological fish, allowing it to be self-sufficient and capable of fast movement (Marchese et al. 2014 ) (Fig.  6 b). The Electrical Engineering and Computer Science Department of the University of Michigan (Ozog et al. 2017 ). The robot adjusts its height through a buoyancy module, and a motor in the tail provides power and adjusts its direction. The flexible part of the robotic fish, designed by the State Key Laboratory of Complex Systems Management and Control at the Institute of Automation, Chinese Academy of Sciences, consists of three joints connected by an aluminum exoskeleton. Each joint is connected to an R/C servo motor that controls the rotation angle of the joint. The rubber caudal fin is connected to the third segment by the peduncle and is crescent-shaped with good coordination (Yu et al. 2016 ) (Fig.  6 c). The robot fish designed by the University of the Chinese Academy of Sciences employs a magnetic actuator as a motor. The propulsion system is characterized by remote control using Bluetooth low power and easy operation through smart devices. By the electromagnetic induction law, the robot fish can swim quickly and turn flexibly. This miniature robot fish could be employed for animal behavior research and special underwater tasks (Chen et al. 2017 ) (Fig.  6 d).

Single motor-driven bionic robot. a Soft robotic fish and diver interface module (Katzschmann et al. 2018 ); b  Details of a soft-bodied robotic fish (Marchese et al. 2014 ); c  Prototype of the robotic fish applied to the underwater robot competition (Yu et al. 2016 ); d  Mechanical design of the robotic fish (Chen et al. 2017 ); e  FILOSE robot fish (Salumäe and Kruusmaa 2013 ); f  Robot fish with a wire-driven active body and compliant tail (Haji and Bamdad 2022 )

The Key Laboratory of Marine Engineering in Shandong Province developed a motor-driven robotic fish with artificial side-line sensors that can help enhance the fish’s maneuverability in dark environments. Artificial sidelines simulate the structure of fish sidelines, offering possibilities for underwater sensing technology and robotic fish control (Salumäe and Kruusmaa 2013 ) (Fig.  6 e). Researchers from Shahrud University of Technology studied a robotic fish with a streamlined drive body and a flexible tail, comprising a network of pressurized liquid-filled chambers embedded in an elastic beam. Viscous fluids with different pressures flow in the channel, producing normal and shear stresses in the channel, which can make the robot fish adapt to different water pressure environments (Haji and Bamdad 2022 ) (Fig.  6 f).

The salamander robot has a modular design comprising seven drive elements and a head element (with the same appearance as the others). The housing of each element includes two symmetrical parts molded with a lightweight polyurethane resin. These components are connected using compatible connectors fixed to the output shaft. All output axes are aligned, so plane motion is produced. To ensure that the robot is waterproof, a custom O-ring robot is used with a total length of 77 cm. Asymmetric friction with the ground, which is required to crawl on the ground correctly, is achieved by fixing a pair of passive wheels to each element, thus ensuring a coordinated transition between swimming and crawling of the robot (Crespi and Ijspeert 2008 ) (Fig.  7 a). The four-legged starfish-shaped soft swimming robot’s flexible and natural buoyancy offers many advantages for tasks such as underwater exploration, sample collection, and marine wildlife observations (Du et al. 2021 ) (Fig.  7 b). In Fig.  7 c, the swinging body of the bionic robotic fish is a multilink mechanism connected by hinges and equipped with multiple motors. In swimming, the required body curve can be acquired by adjusting the relative position of each connecting rod, optimizing its control performance and swimming efficiency compared with a single motor (Korkmaz et al. 2012 ). Inspired by the amphibious tortoise, the mother robot is designed with a spherical body, four legs, and two degrees of freedom. Powered by 4 vector water jets and 10 servo motors, it can walk on land and cruise underwater (Shi et al. 2013 ) (Fig.  7 d). The enhanced 3D printing, low cost, multifunction, high mobility, tortoise-like environmental monitoring, and data acquisition mobile amphibious spherical robot by Beijing Institute of Technology has good amphibious performance (Guo et al. 2018 ) (Fig.  7 e). The cuttlefish robot studied by the University of Nevada researchers is powered by two soft fins of multiple embedded IPMC drive motors connected to an Eco-Flex membrane. The traveling wave is generated on the soft fin by drive, the deformation and blocking force of IPMC on the soft fin are measured, and the actuator is characterized, which can have good wave swimming performance in water (Shen et al. 2020 ) (Fig.  7 f).

Multimotor driven bionic robot. a Salamander (left) and fish (right) robots (Crespi and Ijspeert 2008 ); b  Soft starfish (Du et al. 2021 ); c  Demonstration of the body curve fitting method (Korkmaz et al. 2012 ); d  Prototype of the spherical mother robot (Shi et al. 2013 ); e  Amphibious spherical robot (Guo et al. 2018 ); f  Multimotor driven fish robot (Shen et al. 2020 )

Recently, bionic amphibious robots have developed profoundly in bionic structure design, movement performance, and outdoor workability. Researchers from Harbin Engineering University (Li et al. 2021 ) developed a shape-shifted bionic turtle that can travel in water and walk on land. To enhance the reliability of bionic robots in the future, these robots designed for engineering applications are driven by electric motors and are constantly improved. However, due to their performance limitations, large size, and high power consumption, the size and range of motors have become significant limitations.

2.5 Soft actuator drive

For bionic robots driven by responsive soft actuators, often by imitating the movement patterns of marine organisms, artificial muscles are used to cause propulsion by deformation under control of voltage, and although their power and precision cannot be compared with those of electric motors, responsive soft actuators are stimulated to perform better in terms of high adaptability due to their excellent compliance.

Meanwhile, soft actuator-driven bionic robots are widely used in some miniature marine robots because of their smaller size requirements due to the absence of motors, and soft actuators also have a huge advantage regarding range because they usually consume less power when used compared to motor drives (Gao et al. 2022 ). In 2018, the Precision Engineering Institute designed a new robotic fish with an active and compliant propulsion mechanism, a maximum swimming speed of 2.15 body lengths per second, and a maximum instantaneous turning speed of 269°/s (Shintake et al. 2018 ). In 2014, Marche University of Technology designed a Carregi-shaped swimming robot through a multiphysics simulation environment, which can change from a bone-like motion to a Caran-shaped motion (Praczyk 2014 ).

The Harvard University-designed completely soft octopus robot has all parts of its body made by 3D printing technology and feels slightly slimy to the touch. The soft robot has morphing and cushioning and can travel through small, irregular spaces, which can be useful in the medical, military, and exploration fields (Wehner et al. 2016 ) (Fig.  8 a). The octopus robot, developed by Queen Mary University of London, is made entirely of soft materials and employs a new fluid drive mechanism that allows the robot to push forward, change direction, and rotate around its main axis. In addition, it can use multiple tentacles to grab objects or propel them underwater (Fras et al. 2018 ) (Fig.  8 b).

Soft actuator-driven bionic robot. a Octopus robot (Wehner et al. 2016 ); b  Multitentacle fish robot (Fras et al. 2018 ); c  Soft electronic fish (Li et al. 2017 ); d  Whole body stiffness research (Chen and Jiang 2019 )

The soft-bodied fish developed by Zhejiang University is powered entirely by a soft electroactive structure made of a DE and an ionic conducting hydrogel. The robot fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than a soft responsive material-powered previously reported soft robotic fish (Li et al. 2017 ) (Fig.  8 c). In Fig.  8 d, the flexible robotic fish is a transitional stage between rigid robotic fish and flexible robotic fish, with typical soft materials and traditional driving methods (Chen and Jiang 2019 ). Because the soft material has large elastic deformation, it can be restored to its original shape, and the soft material of the robot fish can be used to protect the actuator and waterproof (Liu and Jiang 2022 ). With the development of bionics and materials science, marine release robots are increasingly driven by a variety of methods, and typical stimulus-responsive actuators include IPMC, SMA, responsive hydrogels, pneumatic structures, chemically responsive expanded fluid networks, and living cells (Bai et al. 2021 ). Using simple principles and widely available materials, the highly integrated electric drive module not only eliminates bulky pumps, pipes, and other equipment but also enables precise control of deformation, while the compact form factor also increases portability.

3 Bionic robot drive mode and control strategy

Traditionally, underwater robots have been classified based on Breder’s fish classification: if a fish generates thrust by bending its body/or caudal fins, the resulting motion is classified as a BCF motion. Fish such as eel-shaped, flesh-capsule, tuna, eel and shark can be categorized into BCF types (Jiao et al. 2022 ). However, suppose that fish uses their mid-fin (including anal, dorsal, and pectoral fins) or paired fins (including ventral and pectoral fins) as propulsion mechanisms, the resulting swimming pattern is classified as a mid-fin or paired fin (MPF) movement (Wang and Wang 2014 ). Regardless of the mode of propulsion used, fish movements are characterized by deformed bodies, fluid forces, and their interactions. Moreover, each mode of motion can be classified by wave frequency as fluctuating and oscillating, as can be seen in fish movements.

Oscillatory motion can be applied when the fish generates propulsion from wave-like motion. Otherwise, if the fish uses a rotation-like motion to obtain thrust, this motion can be classified as oscillatory. These two classifications cannot be separated because oscillatory motion can be derived from the fluctuating motion of shorter wavelengths and vice versa. Eels, which usually use their whole body to produce wave-like motions, can be classified as fluctuating motions, while box fish, which only make their tail fins swing due to body inflexibility, can be classified as oscillatory motions. However, in this fish-based classification, problems emerge in the general classification of animal species (Zhang et al. 2017 ). Especially in robotics, there is no restriction to imitate the motion mechanism and shape of fish. Following the traditional fish classification is still difficult if the motion mechanism and shape of the robot are somewhat different from the target animal.

Thus, in this review, a simplified classification model for the robotics domain is put forward. First, similar to BCF- and MPF-type motions in the fish classification, robots can be classified as body or tail-end anal (BCA) and mid-end or paired anal (MPA) (Wang et al. 2022 ). BCA and MPA are classified based on where the drive occurs relative to the central axis and the direction of robot propulsion. BCA achieves propulsion through drive motion along the central axis. However, in MPA, the driving motion occurs outside the central axis. Similar to the fluctuating oscillation classification of fish, the subcategory is set to fluctuating oscillation motion based on the motion of the robot actuator. Like in the fish classification, the fluctuating motion can be expressed as the fluctuating motion in the actuator. In the same way, oscillatory motion refers to the propulsion structure that rotates on its fixture instead of the wave-like shape.

3.1 Robot drive mode classification

3.1.1 mpf model.

The movement modes of fish are classified by body parts used by fish for propulsion into BCF propulsion mode and MPF propulsion mode (Zhou et al. 2023 ). In MPF propulsion mode, the dorsal, ventral, pectoral, and anal fins are mainly utilized as the main propulsion parts, which can maintain high mobility, stability, and swimming efficiency at low speed. In general, it can achieve accurate six degrees of freedom movement, underwater position holding, and steering, but it is difficult to achieve high-speed swimming and acceleration performance is insufficient. Lampreys, an eel MPF swimming robot developed by the Marine Science Center of Northeastern University, uses 10 TiNi filaments of 250 μm to be energized and heated as a motor (Wu et al. 2013 ). It has a simple structure, no noise, and good stealth performance. The fishtail propulsion of the robot designed in this mode is quieter than the traditional propeller, which is especially important in future naval battles. It can greatly improve stealth ability. Japan developed the first MPF robotic fish that can swim freely underwater; this bionic robotic fish is 650 mm long, 500 mm wide, and 0.64 kg, with floating, diving, turn signal, and other functions, and the smooth shape of the robot fish makes the efficiency of the fishtail propeller up to 80% (Scaradozzi et al. 2017 ). It uses the three-joint bionic tail fin as the only power source, with low power consumption, which can extend the battery life and is suitable for long-term underwater cruises, tracking, and other tasks. In 2021, Osaka University developed a pair of miniature fish out of silicone, which was only the size of a hand but could swim at 0.1 m/s (Xie et al. 2021 ). In 2017, Festo, a German company, developed the pectoral fin bionic robotic fish aqua ray with a body length of 615 mm, a wingspan of 960 mm, and a maximum speed of 0.5 m/s (Saxena and Chauhan 2017 ). The mechanical operation process of forward, backward, differential turning, pitching, and other actions of the bionic robot was completed by experiments and tests, completing the goal of the project. In 2009, the China Academy of Automation developed a small robot dolphin that is 560 mm long and weighs about 3.3 kg, which can complete special tasks such as marching, chasing, and searching (Xia et al. 2023 ). The National University of Defense Technology in China produced a prototype of a bionic pectoral fin powered by multiple fins. In the water tank experiment, the robot’s left and right fins moved simultaneously, with a forward speed of 0.13 m/s and a backward speed of 0.15 m/s. Due to the symmetrical structure and movement of the wave fins of the robot, they could smoothly change the gait from forward to backward without turning and move laterally by sending inward counterpropagation waves.

The Institute of Robotics of Beihang University and the Chinese Academy of Sciences successfully developed SPC-II bionic robotic fish, the first practical application of MPF bionic robotic fish in China (Lou et al. 2017 ). The SPC-II bionic robotic fish is 1.23 m long, with a shiny black body, a total weight of 40 kg, and a maximum diving depth of 5 m. It has a prominent camera above its head that collects location data. The SPC-II bionic robotic fish can move, sink, and float freely and flexibly in waves. The MPF bionic robotic fish robot-ray I, robot-ray II, robot-ray III, and robot-ray IV series were developed by Beihang University (Wu et al. 2021a ). Among them, the best-performing robot-ray IV is 320 mm long with a wingspan of 560 mm. The maximum swimming speed is about 0.16 m/s. Moreover, the robot fish has high underwater speed and better load capacity, and the underwater movement trace is smaller. It can perform quick close-in intercepts, search for enemy divers, highly maneuverable patrols, and track underwater targets at high speeds. The bionic underwater vehicle based on the MPF long-wave propulsion principle has the advantages of high mobility, strong anti-interference ability, and environmental friendliness (Korkmaz et al. 2015 ). Thus, research on this bionic underwater vehicle has a broad market prospect and application value.

3.1.2 BCF model

BCF propulsion mode enables most fish to swim by waving or swinging part of the body and tail fin using eddy currents to push the water behind to use the water reaction force to achieve the forward movement of the fish body. When cruising at high speed, high swimming efficiency can be achieved, generally, more than 80%, and the acceleration and starting performance are good. The bionic bull nose shark designed by Beihang University is a BCF robotic fish (Wang et al. 2021 ). The width of the first-generation bull nose shark robots is 28–46 cm, while that of the second-generation one can reach more than 110 cm. The robot fish is driven by two motors on both sides of the pectoral fin (60W DC motor drive); the flexible pectoral fin comprises silicone rubber material, the main material is made up of relatively low-density glass fiber, and the addition of gyroscopes can achieve autonomous navigation function. A steering engine drives the BCF robot eel developed by the Beijing University of Technology. The fin material is a composite material that can be applied to the eddy current environment with a large water flow (Song et al. 2013 ). The robot shark, developed by St. Mary’s College, University of London, also adopts the BCF drive mode (Watts and McGookin 2014 ). It is larger and adopts silicone fins, and the head’s central processor controls the robot’s movement, which can swim upstream in the rapids. The robot shark simulates a shark’s shape and swimming mode, with little disturbance to the environment and no harm to underwater organisms. The multilink glider robot, developed by the China Academy of Electronics and Information Technology, can swim flexibly and glide efficiently in 3D space and is equipped with the main BCF of a three-degree-of-freedom buoyancy drive system as the main propulsion device for stable propulsion in water (Wang et al. 2021 ) (Fig.  9 a). A Lanzhou Jiaotong University-designed BCF-propelled four-fin bionic prototype based on modular design has high efficiency, rich turning modes, good maneuverability, and high turning speed (Li et al. 2018 ) (Fig.  9 b). Jilin University has developed a carpal bone robot fish with a four-degree-of-freedom tail. The robot fish has two modes of radio control and autonomous swimming. The BCF mode has outstanding performance of high speed and high efficiency (Yan et al. 2008 ) (Fig.  9 c). Ferat University in Turkey has developed a bionic boat-shaped autonomous robotic fish prototype with a double-link tail propulsion mechanism. To simulate the robust swimming gait of fish, a bionic motion control structure based on CPG is adopted. The unidirectional chain CPG network designed is inspired by the neural spinal cord of lampreys and is propelled by BCF. It produces a steady, rhythmic pattern of oscillations underwater (Ay et al. 2018 ) (Fig.  9 d).

BCF mode swimming robot fish. a Main components of the FishBot (Wang et al. 2021 ); b Prototype of the proposed robotic fish (Li et al. 2018 ); c  Mechanical structure of the robotic fish (Yan et al. 2008 ); d  Detailed mechanical configuration of the robotic fish (Ay et al. 2018 )

In 2015, the Harbin Institute of Technology successfully developed a double-jointed Karan-shaped fish robot, code-named 'HRF-I', with a swimming speed of 0.5 m/s (Wang et al. 2015 ). In 2018, the University of Science and Technology of China designed a four-joint bionic robotic fish based on the morphological structure and movement form of the Karan-shaped fish (Zhong et al. 2018 ). In 2016, a BCF model bionic eel robotic fish with eight joints was developed in the United States, and in 2017, Beihang University developed a series of fibular bionic robotic fish with two parallel joints in the tail stalk and tail fin, driven by a two-axis servo motor (Yu et al. 2017 ). The simulated or caudal-fin BCF pulsating underwater thruster, developed by Osaka University in Japan in 2017, has flexible fins on both sides and is driven by 16 DC servo motors via the top fins. The robot fish can realize flexible underwater movements such as surface, diving, steering, pitching, and hovering, which confirms the viability of the application of undulation fin bionic underwater propellers to future underwater robots (Ravalli et al. 2017 ). Several flexible fish species, such as dolphins, sharks, and tuna, swim in BCF mode and can swim with high speed and efficiency. Based on this design, the BCF mode robot produces thrust by bending the torso and swinging the tail fin, leading to high swimming speed, high efficiency, and fast starting performance; thus, the BCF mode is suitable for applications such as long distance and high-speed swimming, instantaneous acceleration, or fast steering (Rajamohamed and Raviraj 2015 ).

3.2 Bionic flutter rigid drive

The fluttering rigid drive mode is a structure of self-excited vibration consisting of skeletal, muscular, and nerve centers (Wang et al. 2021 ). It is the main mode of large aquatic animals with large spreading chord ratios and thickness, such as turtles and penguins. It uses periodic changes in the bending and sinking posture of the up and down swinging forelimbs to regulate the water’s angle of approach. It can be controlled independently of the winging posture to produce forward thrust of the swim front itself, where the forelimbs swing in a process that produces orthogonal directional (negative) drag and lift forces, prompting the body to keep advancing (Todd et al. 2020 ). Although large, this biological body has the advantages of explosive power, high efficiency, stability, low noise, excellent maneuverability, and operational performance. Several theoretical and experimental works have been conducted on marine fluttering organisms by combining bionics from several disciplines. Based on this, a series of bionic flutter wing propulsion devices have been developed with beneficial results. The Chinese Academy of Sciences designed a four-joint robotic fish head, which is a hollow, rigid, and streamlined shell made of molded glass fiber that provides enough space for electromechanical components such as control circuits, sensors, rechargeable batteries, and balancing heavy objects. To duplicate the movement of fish, a series of multilink rigid motors connected by yaw joints are used as the main propulsion mechanism, followed by a slender tail shaft made of polyvinyl chloride and then a polyurethane tail fin with some elasticity. All the rods, driven by DC servo motors, are connected in series to a metal skeleton covered with a flexible waterproof skin that allows for flexible turns in water up to 213° (Su et al. 2014 ) (Fig.  10 a). The robot adopts motor modularity to facilitate loading and unloading. In Fig.  10 b, the Nanyang knife-fish robot contains three independent modules, namely, the buoyancy box module, power cabin module, and wave fin module. Because these modules are designed in a modular manner, these modules can be easily replaced if the design changes or additional features need to be attached (Low 2009 ).

Flutter rigid drive robot. a Slim fish robotic prototypes applied to C-start experiments (Su et al. 2014 ); b  Southern Ocean knife-fish module (Low 2009 ); c  Schematic structure of a G9 series robotic fish (Liu and Hu 2010 ); d  Prototype of biomimetic fish, NAF-I (Chong et al. 2009 )

To investigate the effects of electric motors on robotic fish, the University of Essex in the United Kingdom performed production tests on the G9 series of robotic fish, which are about 52 cm long and have three or four servo motors and two DC motors (Liu and Hu 2010 ) (Fig.  10 c). The servo motor is connected at the tail as three joints; the head is fitted with a DC motor that changes the fish’s center of gravity, and another controls a miniature pump that adjusts the robot’s weight by pumping water. Enhancements in the mechanical structure and skin materials have improved the efficiency and robustness of the robotic fish. The robot fish, NAF-I, weighs about 6.8 kg and is 650 mm long, 100 mm wide, and 260 mm high. It is powered by a 15 V nickel-metal hydride battery, allowing the fish prototype to swim for up to 4 h when fully charged. One DC motor drives the oscillating tail fin, and the other drives the counterweight, and the robot swims in a straight line at a speed of about 0.35 m/s, equivalent to about half a body length per second. It is also confirmed that the greater the thrust of the motor on the robot fish, the faster its swimming speed (Chong et al. 2009 ) (Fig.  10 d). To produce greater thrust, the choice of motor parameters becomes very significant.

Research on bionic flutter drive systems has never stopped. Still, due to the complexity of the drive mechanism and its unique motion characteristics and the different research methods, the forms of flutter wing propulsion are also different (Zhu 2018 ). So far, it has been impossible to conduct a theoretical study for various bionic drive mechanisms because many crucial technical and theoretical problems remain in the research stage, and the technical design of various bionic propulsion systems is still very backward and far from practical application.

3.3 Bionic wave oscillation rigid drive

At present, the main biomimetic fish propulsion systems are BCF models, such as dolphins, which are propelled by the caudal fin, and MPF models, such as manta rays, which are propelled by the pectoral fin.

The propulsion model has high thrust, stability, and maneuverability (Jung et al. 2002 ). It has an excellent performance in fast swimming under hydrostatic conditions and better start and stop functions but poor maneuverability in low-speed turns and turbulent environments. Peking University developed a robot fish consisting of a rigid head, a flexible body, and a tail fin. The hard head houses a control unit, a wireless communication module, and a set of batteries. The battery is placed at the bottom of the head to ensure the vertical stability of the robot while swimming. A pair of pectoral fins are fixed on both sides of the head to ensure the stability of the fins in water. The flexible body comprises three joints, each connected to a servo motor to adjust the deflection angle of the joint. The rubber tail fin is fixed on the third joint and acts with the water flow to move forward in waves (Li et al. 2014a ) (Fig.  11 a). Developed by the Institute of International Education, the HRF is a new type of marine robot with different modes of motion to adapt to the complex marine environment. The motion mode of the hybrid robot fish mainly has two types, namely, sail drive and wave drive. The HRF includes tail fins, wings, steering rubber, collapsible sails, and a hull. In wind-driven mode, the sail is folded, while wave drive is used to drive the hydrofoil up and down with waves to provide power; thus, no extra energy is needed to move forward (Ma et al. 2020 ) (Fig.  11 b).

Wave rigid drive robot. a Prototype of the robotic fish (Li et al. 2014a ); b Prototype of the HRF in wind-driven mode (Ma et al. 2020 ); c  BCF mode swimming style (Chowdhury et al. 2011 ); d  Amphibious snake robot (Kelasidi et al. 2016 )

In 2002, MIT developed the world’s first robotic fish—bionic tuna—which can complete complex movements such as propulsion, turning, and ascent diving (Koch 2002 ). Its forward speed can reach 2 m/s, and the propulsion efficiency is as high as 91%. Building on this, the MIT team developed the reinforced fish Robopike and the steel-like underwater vehicle VCUUV in collaboration with Draper Lab in the United States. In 2016, the birth of these two robotic fish greatly improved the BCF mode propulsion technology (Kumar et al. 2016 ). A hydraulic bionic wave fin prototype is designed at the National University of Defense Technology, comprising a hydraulic pressure source, a hydraulic bionic wave fin principle prototype, and a data acquisition and processing system. The flow variation rule, the function principle of bionic oscillating joint movement, the underwater speed test, and the free navigation propulsion test were performed on the prototype. The National University of Singapore developed a fish-like underwater vehicle integrating fish-like swimming, modular link, and fin movement. The motor is used for simulation of the wave of the fish tail, that is, sinusoidal oscillation. The aim is to duplicate the BCF model’s propulsion technology to swim efficiently over long distances at impressive speeds (Chowdhury et al. 2011 ) (Fig.  11 c). Developed by the Norwegian University of Science and Technology, the amphibious snake robot has similar kinematics whether on land or in water; the snake robot constantly changes its body shape to reduce ground friction or hydrodynamic resistance to achieve forward propulsion, that is, when the snake robot follows a wavy gait pattern, it gains propulsion (Kelasidi et al. 2016 ) (Fig.  11 d).

3.4 Special drive mode

Various underwater organisms drive in different ways, and simulation methods have always been employed to explore their motion mechanism and optimize their motion to guide the design and production of underwater robots. Several strange underwater organisms also bring inspiration to researchers. For example, there is increasing research on underwater jellyfish octopus. The organism is flexible, and only by changing the size of the cavity does it achieve steering and fixed trajectory movement. However, its swimming stability is poor, and the direction is not easy to control, which is a huge issue to solve. Bionic water snake robots, like water snakes, can swim freely in water by swinging their tails. Their movement is flexible and can complete relatively complex task environments. They have good flexibility and freedom in some locations that divers or other underwater vehicles cannot reach because of their appearance. Turtles in water do not have the same slow movement as on the ground: they swim very fast and are very sensitive. Their unique way of propulsion also offers a lot of inspiration to researchers, especially those who study amphibious robot turtles.

3.4.1 Amphibian drive mode

Since the world’s first bionic amphibious robot was designed in 2013, it has gradually developed astonishing achievements. The working environment of the bionic amphibious robot consists of a beach, wetland, underwater, and other complex terrains, and the biological prototype mainly comprises aquatic and terrestrial organisms. Researchers have established many theoretical models such as 'resistance theory', 'slender body theory', and 'inverted pendulum model', but most of them are only applicable to static laboratory environments, and the working environment of bionic amphibious robots is complex and changeable. Thus, it needs to sense the external environment information, parameter change trend, and functional state in real time. In 2010, the Tokyo Institute of Technology Robotics Laboratory designed the serpentine amphibious robot ACM-R5 based on the previously developed HELIX, which had poor performance (Yang and Ma 2010 ). The robot has a 3D motion capability, and each module has a motion mode of two degrees of freedom, capable of pitching and yawing. It has many gaits on land, but its gait in water has not been studied yet. To make serpentine amphibious robots have more flexible mobility in water, the State Key Laboratory of Robotics of the Chinese Academy of Sciences developed a new amphibious robot called EXPLORER-III in 2020, which consists of nine waterproof modular universal units, each with two free-motion modes of pitch and yaw (Zheng et al. 2020 ). The robot has a total length of 117 cm, a trunk diameter of 7.5 cm, and a total mass of 6.75 kg. Since 2016, the State Key Laboratory of Robotics of the Shenyang Institute of Automation, Chinese Academy of Sciences, has conducted extensive research on another serpentine amphibious robot and developed a prototype (Yang et al. 2016 ). The robot is 700 mm long, 320 mm wide, and 150 mm high, with a total mass of 4.995 kg. Moreover, the robot can move at a speed of up to 0.45 m/s in water. Bionic amphibious robots will simplify amphibious drive structures by using soft actuators, improving energy efficiency, sensing the environment, and having a certain ability to make autonomous decisions.

Wheel-propeller-integrated amphibious robots tend to integrate multiple drive units, which can crawl in water and on the ground. Thus, the driving device does not need to be changed; only the mode of motion needs to be changed, which can result in good motion performance on land and in water (Liu and Jiang 2022 ). Thus, research on such robots worldwide has gradually increased, and researchers have achieved some great results. The Mechanical Engineering and Automation major of Beihang University designed an integrated wheel-propeller amphibious robot, which has a simple and compact structure and can realize autonomous movement in two environments (Wu et al. 2021b ). Shenyang Institute of Automation, Chinese Academy of Sciences developed an integrated wheel-propeller amphibious robot with dimensions (L × W × H) of 1.0 m × 0.96 m × 0.2 m. The total weight is 44 kg, the maximum crawling speed is 1 m/s, the maximum swimming speed is 0.7 cm/s, and the maximum working depth is 10 m (You et al. 2010 ). Individual motors drive all drives of the robot, and depending on the operating environment, the movement can be easily switched by rotating the wheel-propeller 90°, but it needs to consume a lot of energy, and the energy of bionic amphibious robots is extremely limited, and the efficiency of energy utilization is low, limiting its application. From the perspective of broadening income sources and reducing expenses, on the one hand, bionic amphibious robots must carry batteries with higher energy density and enhance outdoor energy collection capabilities.

Revealing the movement characteristics of biological prototypes is the premise of bionic design. Due to the rapid development of biology, chemistry, structural science, and other disciplines, research on the driving mechanism of various underwater and land animals has gradually entered the muscle tissue structure and microcell energy utilization process. More accurate mathematical models are required to offer a theoretical basis for designing underwater and land-driven robotic structures. In bionic engineering science, several motion characteristics and swimming mechanisms of aquatic organisms have not been fully explored, such as the effect of dynamic instability on swimming efficiency and the drag reduction function of aquatic organisms (Li et al. 2021 ). Thus, there remains a big gap between most underwater bionic robots and their prototypes. Enhancing the driving efficiency of wave motion and oscillation motion is one of the crucial problems in solving bionic wave motion, but so far, this problem has not been well solved. Thus, investigating the motion characteristics and swimming mechanism of the bionic prototype and applying it to the bionic system, exploring the hydrodynamic factors in the swimming process, and improving the bionic similarity are the key issues to achieving efficient swimming of the model (Serhat, 2022 ).

3.4.2 Bionic water jet soft drive

Aquatic cephalopods such as squid and jellyfish can control the contraction and expansion of the cavity through muscle fibers during swimming, and their movement is in an unstable state of acceleration and deceleration (Zhou et al. 2014 ). At the same time, they are propelled by forces in the opposite direction of the water jet, which enables mollusks such as jellyfish to move axially at extremely high instantaneous speeds and precisely position themselves in slow motion. However, the expansion and contraction of the cavity are not completed, and the air is slowly sucked in and out, leading to discontinuous propulsion and poor movement continuity. From the above theories, the research group of the Liquid Metal Laboratory of the Institute of Physics and Chemistry of the Chinese Academy of Sciences explored the motion characteristics of jellyfish expansion and water absorption, systematically discussed the theory and technology of the liquid metal robot jellyfish integrating the interaction of a fully flexible electromagnetic coil and a magnet for the first time, and designed a bionic robot jellyfish with more natural motion and propulsion (Zhou et al. 2018 ) called RoMan-III. This is driven by a completely soft electromagnetic actuator, which can realize a variety of soft swimming in response to different electrical signals. Based on further conceptual experiments and computational fluid dynamics simulations, Waseda University in Japan systematically explained the response mechanism of the robot jellyfish and various factors controlling its movement behavior, including the formation of vortices and the way of rising, diving, and levitation, and developed a bionic jellyfish with a spherical structure that can float better (Francis et al. 2002 ). By experiment, it was found that this structure can complete the retractable movement of jellyfish more smoothly.

The squid water jet propulsion process principle is as follows: First, the squid outer box membrane expands to form negative pressure, and water fills the chamber. Second, the mantle shrinks sharply after the water jet and funnel are closed. Finally, the air is rapidly ejected from the nozzle, and the body is subjected to a force in the opposite direction of the airflow. The compressed shell of the stingray robotic fish, developed at Nanjing University, is made of photosensitive resin, and the pectoral fin skeleton is composed of 12 carbon fiber rods. The robotic fish uses a thin rubber film to squeeze the water around it as it swims to generate thrust. The oscillation of fin rays causes the fluctuation of pectoral fins, and by controlling the amplitude, frequency, and phase difference between adjacent fins, different harmonic waveforms can be produced (Wang et al. 2014b ) (Fig.  12 a). In Fig.  12 b, the bull nose fish robot simulates the pleural motion and deformation of the bull nose rays. Each side of its internal skeleton comprises three fin-like rays, which are evenly distributed at the base of the fins along the chord. These fins play a significant role in propulsion. The tail fin functions like a lifting rudder, producing power by beating the current to help the pectoral fin float and dive (Cai et al. 2019 ).

Water spraying manta ray robot. a Robotic stingray design (Wang et al. 2014b ); b  Ox nose fish robot (Cai et al. 2019 ); c  Robo-Ray IIs (Kapetanovic et al. 2020 ); d  Underwater robot with elastic skin (Ma et al. 2015 )

California Institute of Technology established a piston jet model by studying the propulsion mechanism of a squid water jet (Wu et al. 2019 ) that used dynamic grid technology to simulate the formation process of vortex rings under different spindle ratios and backgrounds. The reasons for the formation of vortex rings were analyzed, and the consistency of simulation and experimental results was effectively confirmed. Harbin Institute of Technology developed a water film and bionic nozzle based on a cuttlefish jet system (Tian et al. 2022b ). The bending performance of the bionic nozzle was tested at different water temperatures and driving pulse conditions. Researchers used force sensors and high-definition cameras to capture and record the movement of the bionic jet system, effectively confirming the performance characteristics of the bionic jet system (Wang et al. 2017 ). A new bionic manta ray robot was developed by Beihang University. The real flexible deformation of pectoral fins can be well simulated by integrating flexible mechanisms and rigid support into the mechanical structure design of the robot. Second, the CPG control method is used to realize that the controller drives the rhythmic bionic movement, and the flapping wing shoots water to push the body forward and up and down (Kapetanovic et al. 2020 ) (Fig.  12 c). The bionic pectoral fin of the manta ray robot developed by Beihang University can produce an effective angle of attack, and the thrust generated by the interaction with the current can effectively propel the robot fish. The experimental results exhibit that the maximum forward speed of the robot fish can reach 0.43 m/s (0.94 times body length/second) when it is swimming in the tank, and it has good small radius turning maneuverability (Ma et al. 2015 ) (Fig.  12 d).

Due to different conditions, various bionic water jet propulsion systems cannot realize the same movement as real organisms, nor do they have extremely sensitive responses and fast movement ability. However, research on biomimetic water jet propulsion systems is still in its nascent stage: there is no relatively mature biomimetic propulsion system, the types of technologies are relatively small, there are several difficulties to be overcome, and there is a long way to go.

4 Applications

Oceans are vital to life on Earth; they are key to regulating climate and balancing various ecosystems (Park and Kim 2016 ). They are also home to countless creatures and diverse environments. In addition, the oceans are important channels for global transportation. They are indispensable sources of energy. Despite their vital significance, oceans remain underexplored due to their harsh conditions, making exploration impossible with traditional methods. Using underwater vehicles for ocean exploration is becoming increasingly popular as they allow people to conduct safe exploration in extreme environments for long periods. At present, underwater bionic robots are used in many fields, from oil and gas and fisheries to archaeology, search, rescue, and defense (Li et al. 2014c ). In addition, underwater robots are of use in scientific missions, such as mapping water composition and environmental parameters over time and space, exploring the characteristics of the seafloor in terms of depth, morphology, and composition, investigating glacial areas and icebergs, observing biological species in the environment, collecting biological and geological samples, searching for life in the deep ocean, and helping protect the environment from pollution.

4.1 Application status of underwater robots

Since the second half of the twentieth century, underwater robots have begun to assist human exploration of the ocean, and with the continuous advancement of human reach and exploration depth, underwater robots performing various tasks have also been born. In 2017, Professor Yang Canjun of Zhejiang University designed an underwater robot that can automatically clean marine life 100 m below the surface of water. In its first sea test, the robot sent back a 'selfie' video underwater: firmly attached to the wall of the tube, spraying water filled with bubbles, and the accumulated shells were 'swept' away. The robot is specially designed to clean the marine organisms attached to the surface of a steel pipe of an oil drilling platform and has been successfully tested in the Pinghu oil and gas field in the East China Sea. In 2018, the underwater unmanned robot enterprise Yoken Robot launched a new product—BW Space Pro—which is the world’s first underwater UAV with intelligent functions, which is widely used in diving entertainment, underwater shooting, underwater survey, sea fishing, marine environmental protection, marine biological research, aquaculture, underwater archaeology, underwater search and rescue, and other fields. In 2019, Dr. Erik Engeberg of Florida Atlantic University in the United States developed a jellyfish robot that can autonomically shuttle between coral reefs and monitor jellyfish robots at close range. Besides assisting in research, the jellyfish robot can shoulder the task of defending the ocean and serve as a small spearhead in the front line of protecting the environment. In July 2020, the team of Professor Wen Li of Beijing University of Aeronautics and Astronautics and Junzhi, a researcher from the Institute of Automation of the Chinese Academy of Sciences, designed and manufactured an underwater soft robot arm that can be applied to the natural environment of the near shallow sea, with the aim of establishing the kinematic model and rapid solution method of inverse kinematics to realize real-time kinematic control and finally realizing underwater grasp operations in the natural environment of the near shallow sea. With the upgrading and mature application of underwater robot technology, it can not only greatly reduce the risk of manual operation but also improve operation efficiency and reduce the corresponding expenditure cost. Meanwhile, driven by the integration of other innovative technologies, both the comprehensive performance and cost performance levels of underwater robots are continuing to improve, which can better complete the work and is conducive to promoting large-scale development of the industry.

4.2 Natural resource surveys

By duplicating the form of marine organisms, bionic robots can better adapt to harsh environments, such as high pressure, low temperature, and current, at the bottom of the sea. They are usually small in size and light in weight; thus, they can better collect various substances in their original conditions, which is of great significance for the study of natural resources at the bottom of the sea.

Underwater vehicles have been widely used in various marine geoscience research, initially focusing on seafloor mapping but more recently expanding to water column and oceanographic surveys. The first underwater vehicle dedicated to marine was probably the IFREMER AUV, which was used in the early 1980s to map deep-sea manganese nodule fields. A Woods Hole Oceanographic Institution (WHOI) Sentry AUV was used to map the Deepwater Horizon oil spill in the Gulf of Mexico, which resulted in a hydrocarbon plume (Levshonkov et al. 2020 ), using robots carrying detectors to assess its impact on animals and habitats. Many underwater vehicles were deployed in 1995 and 1996 at the Juan de Fuca Ridge in the northwestern United States to detect and map new lava flows (Stenius et al. 2022 ). To use magnetometers to measure young lava flows at 2200 m east longitude, WHOI developed the mixed-material underwater vehicle Nereus for scientific exploration at 11000 m in the deepest part of the ocean. This was almost twice the depth range of the AUV at the time. In 2013, the French National Center for Marine Exploitation built Orca, an unmanned cable-free underwater vehicle with a maximum depth of 6000 m (Gao et al. 2013 ). In 2020, the French National Sea Bomb Development Center cooperated with a company to jointly develop the 'Eret' acoustic remote-control diving robot, which is used for underwater drilling rig inspection, submarine oil rig installation, oil pipeline auxiliary installation, anchor cable reinforcement, and other complex operations. In China, the underwater vehicle was first used in 2022 for subglacial surveys in the Arctic Ocean. Shortly after the scientific survey ship 'Ocean' began the third leg of the expedition, 'Ocean' conducted its first underwater robot operation in the East Pacific Sea for the first time with the underwater robot 'Sea Dragon 2', which was used to observe a rare giant chimney in the 'Bird’s nest' black chimney area and carried a robotic arm used to accurately capture about 7 kg of vulcanized black chimney ventilation samples. 'Hailong 2' relying on accurate dynamic positioning, accurately landed on the seabed in the black chimney area of the 'Bird’s nest' and performed camera observation and measurement of hydrothermal environmental parameters. The discovery marks China as one of the few countries worldwide that can use underwater robots to conduct hydrothermal surveys and sampling studies at mid-ocean ridges. The robot fish has the concealment of integrating into the fish, which can be used to collect information on the fish or guide the fish to schedule the distribution or cluster of the fish according to some algorithms (Marras and Porfiri 2012 ) (Fig.  13 a). Thus, underwater bionic robots may effectively be used in marine environment observation, deep-sea resource exploration and development, and deep-sea and polar scientific investigation.

Application of voice-activated soft machine fish. a Robot fish collect information about shoals of fish (Marras and Porfiri 2012 ); b  Deep-sea exploration (Li et al. 2021 ); c  Underwater positioning (Wang et al. 2020 ); d  Underwater imaging (Katzschmann et al. 2018 )

4.3 Biodiversity research

Through the bionic robot’s similarity in appearance to marine life, marine life can be studied without disturbing its normal activities, enabling close observation of marine life and potentially becoming a new platform for studying and interacting with underwater species (Wang et al. 2020 ). Underwater bionic robots play an important role in marine ecological protection. First, they can be used to collect marine environmental data. Using underwater robots, scientists can obtain detailed geographic images of the ocean and the conditions at the bottom of the ocean. This data is crucial for understanding the health and pollution levels of marine ecosystems. Professor Li Tiefeng’s team at Zhejiang University began research on a bionic deep-sea soft robot based on lionfish. Based on the dispersion and fusion of lionfish head bones in soft tissues, the project team performed the mechanical design of the structure and material of electronic devices and soft matrix and optimized the stress state in the robot body under a high-pressure environment. By designing materials and structures that adjust the devices and software, the robot could withstand a deep-sea pressure of 10000 m without a pressure-resistant shell and successfully conducted exploration missions in the Mariana Trench (Li et al. 2021 ) (Fig.  13 b). Underwater robots can also be used to monitor the population and activity areas of marine life. With cameras and sensors, scientists can observe and record the behavior of many aquatic organisms in real time, providing evidence for their conservation.

In addition to data collection and monitoring, ROV maps can help protect marine life. They can remove debris and harmful substances from the ocean. Many marine creatures often die by ingesting waste. The underwater vehicle can collect this waste through its robotic arm and bring it to a safe location for disposal. Some underwater robot maps can even perform deep seabed cleanup operations to help restore the health of the ocean (Wang et al. 2002 ) (Fig.  13 c). The University of Icahnx developed a new kind of robotic fish for detecting pollution in river water and drawing 3D pollution maps of the river (Gomatam et al. 2012 ). Each robotic fish is about 50 cm long, 15 cm high, and 12 cm wide. Each is equipped with pollution detection sensors and Global Positioning System (GPS), can 'smell' harmful substances in the water, and can work together, even if there is no one to direct. When they 'sniff out' the harmful substances in the water, they communicate with each other through a Wi-Fi wireless connection. The GPS navigation system allows them to swim freely without human operation, and once they find pollutants, they will send an alert to the environmental protection department personnel (Skorohod et al. 2020 ).

Biosensors were first deployed on an underwater robot when an NERC autonomous submersible AUV was fitted with an in situ dissolved manganese analyzer (Skorohod et al. 2020 ). This deployment showed how an autonomous underwater robot carrying a biosensor could detect small-scale changes in species distribution that traditional sampling methods could not address. Since then, chemical sensors on underwater robots used for marine purposes have been used mainly to search the water column for active hydrothermal columns and to study species distributions, and a suite of sensors for detecting hazardous liquid spills have been deployed on underwater robots in the North Sea Sleipner project for frequent, high time scale studies of areas of potential spills to protect the ecological environment (Tran and Park 2020 ). By application of underwater robot mapping, people can better protect the diversity of marine ecosystems and marine life. They help people understand and solve the problems of the marine environment and provide a guarantee for the rational use of marine resources.

4.4 Underwater imaging

There is an increasing demand for exploration of the seabed environment, and the imaging requirements for marine resources and the underwater world are also getting higher and higher (Liang et al. 2010 ). Due to the uncertainty of the underwater environment, such as interference of the current and limited sensing ability, conventional underwater navigation equipment has limitations; thus, bionic robots designed for different underwater environments have great advantages.

The bionic underwater foot robot studied by the National Metrology Institute of Japan (Maeda et al. 2020 ) imitates the appearance and behavior of crabs and can walk and jump underwater. Compared to traditional AUV and ROV, it is better adapted to complex underwater terrains and has a higher affinity for underwater organisms. Due to their bionic appearance, the natural movements of underwater creatures can be well imaged. National Institute of Ocean Technology (Ramesh et al. 2017 ) used the bionic fish REMUS to map the habitat at 1–2 m water depth in the Juan Strait in the northern United States. It used underwater video data for ground truth measurements. Underwater robots have been used to map various seafloor morphological features, including under ice sheets inaccessible to research ships. For instance, State Marine Technical University (Siek and Sakovich 2019 ) used the underwater vehicle NERC Autosub3 to investigate the retreat of the Pine Island Glacier (PIG) in West Antarctica. The robot performed six missions in 94 h, collecting 510 km of orbital data under the PIG ice shelf 50 km above the ice surface.

Underwater vehicles are also being used to image sedimentary features in submarine canyons. The University of Kanagawa used an underwater vehicle carrying a high-resolution multibeam waveform acoustic system (0.7 m lateral resolution) and a submarine profiler (0.1 m vertical resolution) to conduct underwater imaging experiments, collecting data from La Jolla Canyon on the Southern California coast. To understand the processes that produce observational patterns on a scale comparable to the surface (Tsukioka et al. 2002 ), the Science and Technology on Underwater Vehicle Laboratory used underwater robotic fish diving to collect deep-sea data and provide vibration core samples for sediment dating (Liu et al. 2020 ) (Fig.  13 d). In the article on acoustically controlled soft robotic fish studied by the University of Zagreb (Kapetanovic et al. 2020 ), it is possible to approach underwater organisms without disturbing their normal life and to image underwater organisms and underwater landscapes through shape features similar to those of fish (Katzschmann et al. 2018 ).

When conducting underwater imaging, conventional underwater vehicles have higher accuracy in the tangential direction of the seabed and lower accuracy in the vertical direction of the seabed. In comparison, underwater bionic robots have lower accuracy in the tangential direction of the seabed and higher accuracy in the vertical direction of the seabed (Wang et al. 2014a ). Moreover, the underwater bionic robot has high stability and adaptability to the seabed environment, and the combination of the two can obtain higher-quality underwater imaging maps.

4.5 Underwater search and rescue

Underwater robots can be used to check whether explosives are installed on dams and bridge piers, remote-reconnaissance structural conditions or dangerous goods, and closely inspect underwater evidence. In 2010, underwater robots could walk at 3–6 km/h in the deepest underwater world of 6000 m (Brown and Clark  2010 ). The forward-looking and downward-looking radar gives it 'good eyesight'. The accompanying camera, video camera, and precise navigation system allow it to 'overlook'. The underwater robot WHOI provided in 2012 took just a few days to find the wreckage of an Air France flight in 4000 km 2 of ocean after two years of fruitless searching by various ships and aircraft. Underwater robots have great potential and application value in rescue missions. When encountering dangerous situations, underwater bionic robots can play a greater role in on-site situation assessment and positioning, providing important information for the next step. Through the underwater high-definition camera group, sonar, and a variety of sensors carried by the underwater robot itself, rescue workers can grasp the water depth and temperature on the shore. They can determine the obstacles in the water and remove the danger of entering the water (Wang et al. 2019c ). In salvage and other operations, the underwater robot can quickly locate the location of underwater objects. Armed with this information, commanders can better formulate a reasonable and efficient rescue plan. Another major advantage of underwater robots lies in search and rescue missions. In dangerous waters such as rapids and low temperatures, it can take the lead in entering underwater areas that rescuers cannot reach to detect the location and situation of trapped people. The robot operator can control the movement of the robot by manipulating the handle or wireless sensing device on the shore. Carrying tools such as robotic arms can also assist rescue workers in completing tasks such as clearing and salvaging. The water environment where the danger occurs is not always ideal, and low visibility is one of the most significant problems. Bionic fluorescent robot fish can provide rescuers with a light source, and rescuers can also determine the location of the target and search for risks by referring to the umbilical cable connected to the underwater robot (Asadnia et al. 2015 ). The emergence of underwater robots makes underwater rescue work safer and more efficient.

5 Summary and outlook

From the above summarized research results, it can be observed that research on bionic underwater robots has grown considerably. Rapid turning, path tracking, autonomous operation, and other actions have been achieved on some prototypes, and there is a great improvement in speed and mobility, but there is still a very obvious gap with real fish. Underwater bionic robot development is high-end manufacturing industry supported by the Chinese government and plays the role of a 'strategic commanding height'. In China and abroad, a series of work has been conducted on the mechanical structure design, materials, and control methods of underwater bionic robots, and the related research has grown considerably. Due to the complexity of underwater, the mechanical structure design and control technology of underwater robots still require further optimization and improvement to truly achieve a life-like system that integrates the structure and biological characteristics. By enhancing the characteristics of underwater robots, such as self-control and self-perception, and through the coordinated control of robot systems, underwater robots can better integrate into the underwater environment to complete the work, pursue sustainability on the road to development, and make this technology more mature.

Research on bionic underwater robots has become more in-depth and expanded, and some prototypes have realized multimodal motion, fast turning, path tracking, autonomous operation, and other actions, which have greatly improved in speed and maneuverability. However, there is still a very obvious gap with real fish. In the future, bionic underwater robots should be developed into autonomous, intelligent, and collaborative tools. To further improve the performance of the bionic underwater robot system, further work should be conducted in the following main research directions: (1) Mechanism design and optimization. Most bionic underwater robots are driven by motors. Research can be conducted in terms of streamlined low-resistance shapes, intelligent driving materials, and rigid and flexible coupling efficient transmission mechanisms to improve the motion performance of bionic underwater robots. (2) Underwater environment perception and modeling are significant for bionic underwater robots to perform underwater tasks. Information fusion technology of various sensors can be examined and combined with the technology to conduct underwater environment modeling and improve the autonomy intelligence of bionic underwater robots. (3) Intelligent control methods, such as artificial intelligence, are a hot field right now. Some artificial intelligence technologies, such as reinforcement learning and transfer learning, can be applied to the intelligent control of bionic underwater robots so that they can learn various motor skills independently. (4) Multibionic underwater robot cooperation. In nature, fish is often in the form of clusters for foraging, defense, and cruising. The use of multiple bionic underwater vehicles to form a cooperative system is helpful in improving operational efficiency. Due to the complexity of underwater, the particularity of the propulsion mechanism, and the bottleneck of underwater communication, sensing, positioning, and other technologies, the collaboration of multibionic underwater robots will be a very challenging direction.

Due to the complexity of the marine environment, underwater bionic robots will face problems such as the drastic change in water velocity, the difference in pressure under different water depths, and their waterproofing, which poses a great challenge to the structural design of robots. To address these issues, the structure of the future underwater bionic robot needs to be more detailed and more lightweight, and the application of materials should also meet the requirements of the underwater environment. Miniaturization is the current trend of robot development because small structures are easier to adapt to the environment, reduce the contact area, and thus reduce the impact of underwater pressure on the machine structure to a greater extent. The most prominent point is that miniaturized robots are closer to the physiological structure of marine organisms and fundamentally realize the bionic effect rather than just the imitation of appearance. Marine space is generally unsuitable for human survival, and large-scale development and utilization of marine resources have a great dependence on robotics technology. Replacing humans with robots to promote and realize unmanned marine equipment has far-reaching strategic significance. Thus, future bionic underwater robots should be further developed, mainly in the direction of autonomy, intelligence, and synergy, to enhance the performance of bionic underwater robotic systems.

Availability of data and materials

The data and materials used to support the findings of this study are included in the article.

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Acknowledgements

This work was supported by grants from the National Natural Science Foundation of China (Grant Nos. 62201537 and U20A20194), the Natural Science Foundation of Shandong Province (Grant No. ZR2022QF008), and the Central University Basic Research Fund of China (Grant No. 202312035). We thank Yanyue Teng at the Ocean University of China for the useful discussion on the soft actuator drive part of this review.

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Zhongao Cui, Liao Li, Yuhang Wang, Zhiwei Zhong & Junyang Li

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Zhongao Cui and Liao Li performed the literature survey, drafted the manuscript and revised it critically for the key content. Yuhang Wang conducted literature research and content verification. Zhiwei Zhong carried out the document sorting and figure modification. Junyang Li is the corresponding author, responsible for organizing the manuscript sequence alignment, proofreading and revising the manuscript, and giving the final approval of the version to be published. All authors read and approved the final manuscript.

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Cui, Z., Li, L., Wang, Y. et al. Review of research and control technology of underwater bionic robots. Intell. Mar. Technol. Syst. 1 , 7 (2023). https://doi.org/10.1007/s44295-023-00010-3

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Received : 24 August 2023

Revised : 12 September 2023

Accepted : 14 September 2023

Published : 18 October 2023

DOI : https://doi.org/10.1007/s44295-023-00010-3

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