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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

APA7 Style resources

APA Style Blog - for those harder to find answers

1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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Literature reviews, what is a literature review, learning more about how to do a literature review.

• Planning the Review
• The Research Question
• Choosing Where to Search
• Organizing the Review
• Writing the Review

A literature review is a review and synthesis of existing research on a topic or research question. A literature review is meant to analyze the scholarly literature, make connections across writings and identify strengths, weaknesses, trends, and missing conversations. A literature review should address different aspects of a topic as it relates to your research question. A literature review goes beyond a description or summary of the literature you have read. 

• Sage Research Methods Core Collection This link opens in a new window SAGE Research Methods supports research at all levels by providing material to guide users through every step of the research process. SAGE Research Methods is the ultimate methods library with more than 1000 books, reference works, journal articles, and instructional videos by world-leading academics from across the social sciences, including the largest collection of qualitative methods books available online from any scholarly publisher. – Publisher

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Research Methods

• Getting Started
• Literature Review Research
• Research Design
• Research Design By Discipline
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• Teaching with SAGE Research Methods

Literature Review

• What is a Literature Review?
• What is NOT a Literature Review?
• Purposes of a Literature Review
• Types of Literature Reviews
• Literature Reviews vs. Systematic Reviews
• Systematic vs. Meta-Analysis

Literature Review  is a comprehensive survey of the works published in a particular field of study or line of research, usually over a specific period of time, in the form of an in-depth, critical bibliographic essay or annotated list in which attention is drawn to the most significant works.

Also, we can define a literature review as the collected body of scholarly works related to a topic:

• Summarizes and analyzes previous research relevant to a topic
• Includes scholarly books and articles published in academic journals
• Can be an specific scholarly paper or a section in a research paper

The objective of a Literature Review is to find previous published scholarly works relevant to an specific topic

• Help gather ideas or information
• Keep up to date in current trends and findings
• Help develop new questions

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Helps focus your own research questions or problems
• Discovers relationships between research studies/ideas.
• Suggests unexplored ideas or populations
• Identifies major themes, concepts, and researchers on a topic.
• Tests assumptions; may help counter preconceived ideas and remove unconscious bias.
• Identifies critical gaps, points of disagreement, or potentially flawed methodology or theoretical approaches.
• Indicates potential directions for future research.

All content in this section is from Literature Review Research from Old Dominion University 

Keep in mind the following, a literature review is NOT:

Not an essay 

Not an annotated bibliography  in which you summarize each article that you have reviewed.  A literature review goes beyond basic summarizing to focus on the critical analysis of the reviewed works and their relationship to your research question.

Not a research paper   where you select resources to support one side of an issue versus another.  A lit review should explain and consider all sides of an argument in order to avoid bias, and areas of agreement and disagreement should be highlighted.

A literature review serves several purposes. For example, it

• provides thorough knowledge of previous studies; introduces seminal works.
• helps focus one’s own research topic.
• identifies a conceptual framework for one’s own research questions or problems; indicates potential directions for future research.
• suggests previously unused or underused methodologies, designs, quantitative and qualitative strategies.
• identifies gaps in previous studies; identifies flawed methodologies and/or theoretical approaches; avoids replication of mistakes.
• helps the researcher avoid repetition of earlier research.
• suggests unexplored populations.
• determines whether past studies agree or disagree; identifies controversy in the literature.
• tests assumptions; may help counter preconceived ideas and remove unconscious bias.

As Kennedy (2007) notes*, it is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the original studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally that become part of the lore of field. In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews.

Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are several approaches to how they can be done, depending upon the type of analysis underpinning your study. Listed below are definitions of types of literature reviews:

Argumentative Review      This form examines literature selectively in order to support or refute an argument, deeply imbedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to to make summary claims of the sort found in systematic reviews.

Integrative Review      Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication.

Historical Review      Few things rest in isolation from historical precedent. Historical reviews are focused on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review      A review does not always focus on what someone said [content], but how they said it [method of analysis]. This approach provides a framework of understanding at different levels (i.e. those of theory, substantive fields, research approaches and data collection and analysis techniques), enables researchers to draw on a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection and data analysis, and helps highlight many ethical issues which we should be aware of and consider as we go through our study.

Systematic Review      This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyse data from the studies that are included in the review. Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?"

Theoretical Review      The purpose of this form is to concretely examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review help establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

* Kennedy, Mary M. "Defining a Literature."  Educational Researcher  36 (April 2007): 139-147.

All content in this section is from The Literature Review created by Dr. Robert Larabee USC

Robinson, P. and Lowe, J. (2015),  Literature reviews vs systematic reviews.  Australian and New Zealand Journal of Public Health, 39: 103-103. doi: 10.1111/1753-6405.12393

What's in the name? The difference between a Systematic Review and a Literature Review, and why it matters . By Lynn Kysh from University of Southern California

Systematic review or meta-analysis?

A  systematic review  answers a defined research question by collecting and summarizing all empirical evidence that fits pre-specified eligibility criteria.

A  meta-analysis  is the use of statistical methods to summarize the results of these studies.

Systematic reviews, just like other research articles, can be of varying quality. They are a significant piece of work (the Centre for Reviews and Dissemination at York estimates that a team will take 9-24 months), and to be useful to other researchers and practitioners they should have:

• clearly stated objectives with pre-defined eligibility criteria for studies
• explicit, reproducible methodology
• a systematic search that attempts to identify all studies
• assessment of the validity of the findings of the included studies (e.g. risk of bias)
• systematic presentation, and synthesis, of the characteristics and findings of the included studies

Not all systematic reviews contain meta-analysis. 

Meta-analysis is the use of statistical methods to summarize the results of independent studies. By combining information from all relevant studies, meta-analysis can provide more precise estimates of the effects of health care than those derived from the individual studies included within a review.  More information on meta-analyses can be found in  Cochrane Handbook, Chapter 9 .

A meta-analysis goes beyond critique and integration and conducts secondary statistical analysis on the outcomes of similar studies.  It is a systematic review that uses quantitative methods to synthesize and summarize the results.

An advantage of a meta-analysis is the ability to be completely objective in evaluating research findings.  Not all topics, however, have sufficient research evidence to allow a meta-analysis to be conducted.  In that case, an integrative review is an appropriate strategy. 

Some of the content in this section is from Systematic reviews and meta-analyses: step by step guide created by Kate McAllister.

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2.3 Reviewing the Research Literature

Learning objectives.

• Define the research literature in psychology and give examples of sources that are part of the research literature and sources that are not.
• Describe and use several methods for finding previous research on a particular research idea or question.

Reviewing the research literature means finding, reading, and summarizing the published research relevant to your question. An empirical research report written in American Psychological Association (APA) style always includes a written literature review, but it is important to review the literature early in the research process for several reasons.

• It can help you turn a research idea into an interesting research question.
• It can tell you if a research question has already been answered.
• It can help you evaluate the interestingness of a research question.
• It can give you ideas for how to conduct your own study.
• It can tell you how your study fits into the research literature.

What Is the Research Literature?

The research literature in any field is all the published research in that field. The research literature in psychology is enormous—including millions of scholarly articles and books dating to the beginning of the field—and it continues to grow. Although its boundaries are somewhat fuzzy, the research literature definitely does not include self-help and other pop psychology books, dictionary and encyclopedia entries, websites, and similar sources that are intended mainly for the general public. These are considered unreliable because they are not reviewed by other researchers and are often based on little more than common sense or personal experience. Wikipedia contains much valuable information, but the fact that its authors are anonymous and its content continually changes makes it unsuitable as a basis of sound scientific research. For our purposes, it helps to define the research literature as consisting almost entirely of two types of sources: articles in professional journals, and scholarly books in psychology and related fields.

Professional Journals

Professional journals are periodicals that publish original research articles. There are thousands of professional journals that publish research in psychology and related fields. They are usually published monthly or quarterly in individual issues, each of which contains several articles. The issues are organized into volumes, which usually consist of all the issues for a calendar year. Some journals are published in hard copy only, others in both hard copy and electronic form, and still others in electronic form only.

Most articles in professional journals are one of two basic types: empirical research reports and review articles. Empirical research reports describe one or more new empirical studies conducted by the authors. They introduce a research question, explain why it is interesting, review previous research, describe their method and results, and draw their conclusions. Review articles summarize previously published research on a topic and usually present new ways to organize or explain the results. When a review article is devoted primarily to presenting a new theory, it is often referred to as a theoretical article .

Figure 2.6 Small Sample of the Thousands of Professional Journals That Publish Research in Psychology and Related Fields

Most professional journals in psychology undergo a process of peer review . Researchers who want to publish their work in the journal submit a manuscript to the editor—who is generally an established researcher too—who in turn sends it to two or three experts on the topic. Each reviewer reads the manuscript, writes a critical review, and sends the review back to the editor along with his or her recommendations. The editor then decides whether to accept the article for publication, ask the authors to make changes and resubmit it for further consideration, or reject it outright. In any case, the editor forwards the reviewers’ written comments to the researchers so that they can revise their manuscript accordingly. Peer review is important because it ensures that the work meets basic standards of the field before it can enter the research literature.

Scholarly Books

Scholarly books are books written by researchers and practitioners mainly for use by other researchers and practitioners. A monograph is written by a single author or a small group of authors and usually gives a coherent presentation of a topic much like an extended review article. Edited volumes have an editor or a small group of editors who recruit many authors to write separate chapters on different aspects of the same topic. Although edited volumes can also give a coherent presentation of the topic, it is not unusual for each chapter to take a different perspective or even for the authors of different chapters to openly disagree with each other. In general, scholarly books undergo a peer review process similar to that used by professional journals.

Literature Search Strategies

Using psycinfo and other databases.

The primary method used to search the research literature involves using one or more electronic databases. These include Academic Search Premier, JSTOR, and ProQuest for all academic disciplines, ERIC for education, and PubMed for medicine and related fields. The most important for our purposes, however, is PsycINFO , which is produced by the APA. PsycINFO is so comprehensive—covering thousands of professional journals and scholarly books going back more than 100 years—that for most purposes its content is synonymous with the research literature in psychology. Like most such databases, PsycINFO is usually available through your college or university library.

PsycINFO consists of individual records for each article, book chapter, or book in the database. Each record includes basic publication information, an abstract or summary of the work, and a list of other works cited by that work. A computer interface allows entering one or more search terms and returns any records that contain those search terms. (These interfaces are provided by different vendors and therefore can look somewhat different depending on the library you use.) Each record also contains lists of keywords that describe the content of the work and also a list of index terms. The index terms are especially helpful because they are standardized. Research on differences between women and men, for example, is always indexed under “Human Sex Differences.” Research on touching is always indexed under the term “Physical Contact.” If you do not know the appropriate index terms, PsycINFO includes a thesaurus that can help you find them.

Given that there are nearly three million records in PsycINFO, you may have to try a variety of search terms in different combinations and at different levels of specificity before you find what you are looking for. Imagine, for example, that you are interested in the question of whether women and men differ in terms of their ability to recall experiences from when they were very young. If you were to enter “memory for early experiences” as your search term, PsycINFO would return only six records, most of which are not particularly relevant to your question. However, if you were to enter the search term “memory,” it would return 149,777 records—far too many to look through individually. This is where the thesaurus helps. Entering “memory” into the thesaurus provides several more specific index terms—one of which is “early memories.” While searching for “early memories” among the index terms returns 1,446 records—still too many too look through individually—combining it with “human sex differences” as a second search term returns 37 articles, many of which are highly relevant to the topic.

Depending on the vendor that provides the interface to PsycINFO, you may be able to save, print, or e-mail the relevant PsycINFO records. The records might even contain links to full-text copies of the works themselves. (PsycARTICLES is a database that provides full-text access to articles in all journals published by the APA.) If not, and you want a copy of the work, you will have to find out if your library carries the journal or has the book and the hard copy on the library shelves. Be sure to ask a librarian if you need help.

Using Other Search Techniques

In addition to entering search terms into PsycINFO and other databases, there are several other techniques you can use to search the research literature. First, if you have one good article or book chapter on your topic—a recent review article is best—you can look through the reference list of that article for other relevant articles, books, and book chapters. In fact, you should do this with any relevant article or book chapter you find. You can also start with a classic article or book chapter on your topic, find its record in PsycINFO (by entering the author’s name or article’s title as a search term), and link from there to a list of other works in PsycINFO that cite that classic article. This works because other researchers working on your topic are likely to be aware of the classic article and cite it in their own work. You can also do a general Internet search using search terms related to your topic or the name of a researcher who conducts research on your topic. This might lead you directly to works that are part of the research literature (e.g., articles in open-access journals or posted on researchers’ own websites). The search engine Google Scholar is especially useful for this purpose. A general Internet search might also lead you to websites that are not part of the research literature but might provide references to works that are. Finally, you can talk to people (e.g., your instructor or other faculty members in psychology) who know something about your topic and can suggest relevant articles and book chapters.

What to Search For

When you do a literature review, you need to be selective. Not every article, book chapter, and book that relates to your research idea or question will be worth obtaining, reading, and integrating into your review. Instead, you want to focus on sources that help you do four basic things: (a) refine your research question, (b) identify appropriate research methods, (c) place your research in the context of previous research, and (d) write an effective research report. Several basic principles can help you find the most useful sources.

First, it is best to focus on recent research, keeping in mind that what counts as recent depends on the topic. For newer topics that are actively being studied, “recent” might mean published in the past year or two. For older topics that are receiving less attention right now, “recent” might mean within the past 10 years. You will get a feel for what counts as recent for your topic when you start your literature search. A good general rule, however, is to start with sources published in the past five years. The main exception to this rule would be classic articles that turn up in the reference list of nearly every other source. If other researchers think that this work is important, even though it is old, then by all means you should include it in your review.

Second, you should look for review articles on your topic because they will provide a useful overview of it—often discussing important definitions, results, theories, trends, and controversies—giving you a good sense of where your own research fits into the literature. You should also look for empirical research reports addressing your question or similar questions, which can give you ideas about how to operationally define your variables and collect your data. As a general rule, it is good to use methods that others have already used successfully unless you have good reasons not to. Finally, you should look for sources that provide information that can help you argue for the interestingness of your research question. For a study on the effects of cell phone use on driving ability, for example, you might look for information about how widespread cell phone use is, how frequent and costly motor vehicle crashes are, and so on.

How many sources are enough for your literature review? This is a difficult question because it depends on how extensively your topic has been studied and also on your own goals. One study found that across a variety of professional journals in psychology, the average number of sources cited per article was about 50 (Adair & Vohra, 2003). This gives a rough idea of what professional researchers consider to be adequate. As a student, you might be assigned a much lower minimum number of references to use, but the principles for selecting the most useful ones remain the same.

Key Takeaways

• The research literature in psychology is all the published research in psychology, consisting primarily of articles in professional journals and scholarly books.
• Early in the research process, it is important to conduct a review of the research literature on your topic to refine your research question, identify appropriate research methods, place your question in the context of other research, and prepare to write an effective research report.
• There are several strategies for finding previous research on your topic. Among the best is using PsycINFO, a computer database that catalogs millions of articles, books, and book chapters in psychology and related fields.
• Practice: Use the techniques discussed in this section to find 10 journal articles and book chapters on one of the following research ideas: memory for smells, aggressive driving, the causes of narcissistic personality disorder, the functions of the intraparietal sulcus, or prejudice against the physically handicapped.

Adair, J. G., & Vohra, N. (2003). The explosion of knowledge, references, and citations: Psychology’s unique response to a crisis. American Psychologist, 58 , 15–23.

Research Methods in Psychology Copyright © 2016 by University of Minnesota is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Organizing Your Social Sciences Research Paper

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Reading a Scholarly Article or Research Paper

Identifying a research problem to investigate usually requires a preliminary search for and critical review of the literature in order to gain an understanding about how scholars have examined a topic. Scholars rarely structure research studies in a way that can be followed like a story; they are complex and detail-intensive and often written in a descriptive and conclusive narrative form. However, in the social and behavioral sciences, journal articles and stand-alone research reports are generally organized in a consistent format that makes it easier to compare and contrast studies and to interpret their contents.

General Reading Strategies

W hen you first read an article or research paper, focus on asking specific questions about each section. This strategy can help with overall comprehension and with understanding how the content relates [or does not relate] to the problem you want to investigate. As you review more and more studies, the process of understanding and critically evaluating the research will become easier because the content of what you review will begin to coalescence around common themes and patterns of analysis. Below are recommendations on how to read each section of a research paper effectively. Note that the sections to read are out of order from how you will find them organized in a journal article or research paper.

1.  Abstract

The abstract summarizes the background, methods, results, discussion, and conclusions of a scholarly article or research paper. Use the abstract to filter out sources that may have appeared useful when you began searching for information but, in reality, are not relevant. Questions to consider when reading the abstract are:

• Is this study related to my question or area of research?
• What is this study about and why is it being done ?
• What is the working hypothesis or underlying thesis?
• What is the primary finding of the study?
• Are there words or terminology that I can use to either narrow or broaden the parameters of my search for more information?

2.  Introduction

If, after reading the abstract, you believe the paper may be useful, focus on examining the research problem and identifying the questions the author is trying to address. This information is usually located within the first few paragraphs of the introduction or in the concluding paragraph. Look for information about how and in what way this relates to what you are investigating. In addition to the research problem, the introduction should provide the main argument and theoretical framework of the study and, in the last paragraphs of the introduction, describe what the author(s) intend to accomplish. Questions to consider when reading the introduction include:

• What is this study trying to prove or disprove?
• What is the author(s) trying to test or demonstrate?
• What do we already know about this topic and what gaps does this study try to fill or contribute a new understanding to the research problem?
• Why should I care about what is being investigated?
• Will this study tell me anything new related to the research problem I am investigating?

3.  Literature Review

The literature review describes and critically evaluates what is already known about a topic. Read the literature review to obtain a big picture perspective about how the topic has been studied and to begin the process of seeing where your potential study fits within the domain of prior research. Questions to consider when reading the literature review include:

• W hat other research has been conducted about this topic and what are the main themes that have emerged?
• What does prior research reveal about what is already known about the topic and what remains to be discovered?
• What have been the most important past findings about the research problem?
• How has prior research led the author(s) to conduct this particular study?
• Is there any prior research that is unique or groundbreaking?
• Are there any studies I could use as a model for designing and organizing my own study?

4.  Discussion/Conclusion

The discussion and conclusion are usually the last two sections of text in a scholarly article or research report. They reveal how the author(s) interpreted the findings of their research and presented recommendations or courses of action based on those findings. Often in the conclusion, the author(s) highlight recommendations for further research that can be used to develop your own study. Questions to consider when reading the discussion and conclusion sections include:

• What is the overall meaning of the study and why is this important? [i.e., how have the author(s) addressed the " So What? " question].
• What do you find to be the most important ways that the findings have been interpreted?
• What are the weaknesses in their argument?
• Do you believe conclusions about the significance of the study and its findings are valid?
• What limitations of the study do the author(s) describe and how might this help formulate my own research?
• Does the conclusion contain any recommendations for future research?

5.  Methods/Methodology

The methods section describes the materials, techniques, and procedures for gathering information used to examine the research problem. If what you have read so far closely supports your understanding of the topic, then move on to examining how the author(s) gathered information during the research process. Questions to consider when reading the methods section include:

• Did the study use qualitative [based on interviews, observations, content analysis], quantitative [based on statistical analysis], or a mixed-methods approach to examining the research problem?
• What was the type of information or data used?
• Could this method of analysis be repeated and can I adopt the same approach?
• Is enough information available to repeat the study or should new data be found to expand or improve understanding of the research problem?

6.  Results

After reading the above sections, you should have a clear understanding of the general findings of the study. Therefore, read the results section to identify how key findings were discussed in relation to the research problem. If any non-textual elements [e.g., graphs, charts, tables, etc.] are confusing, focus on the explanations about them in the text. Questions to consider when reading the results section include:

• W hat did the author(s) find and how did they find it?
• Does the author(s) highlight any findings as most significant?
• Are the results presented in a factual and unbiased way?
• Does the analysis of results in the discussion section agree with how the results are presented?
• Is all the data present and did the author(s) adequately address gaps?
• What conclusions do you formulate from this data and does it match with the author's conclusions?

7.  References

The references list the sources used by the author(s) to document what prior research and information was used when conducting the study. After reviewing the article or research paper, use the references to identify additional sources of information on the topic and to examine critically how these sources supported the overall research agenda. Questions to consider when reading the references include:

• Do the sources cited by the author(s) reflect a diversity of disciplinary viewpoints, i.e., are the sources all from a particular field of study or do the sources reflect multiple areas of study?
• Are there any unique or interesting sources that could be incorporated into my study?
• What other authors are respected in this field, i.e., who has multiple works cited or is cited most often by others?
• What other research should I review to clarify any remaining issues or that I need more information about?

NOTE :  A final strategy in reviewing research is to copy and paste the title of the source [journal article, book, research report] into Google Scholar . If it appears, look for a "cited by" followed by a hyperlinked number [e.g., Cited by 45]. This number indicates how many times the study has been subsequently cited in other, more recently published works. This strategy, known as citation tracking, can be an effective means of expanding your review of pertinent literature based on a study you have found useful and how scholars have cited it. The same strategies described above can be applied to reading articles you find in the list of cited by references.

Reading Tip

Specific Reading Strategies

Effectively reading scholarly research is an acquired skill that involves attention to detail and an ability to comprehend complex ideas, data, and theoretical concepts in a way that applies logically to the research problem you are investigating. Here are some specific reading strategies to consider.

As You are Reading

• Focus on information that is most relevant to the research problem; skim over the other parts.
• As noted above, read content out of order! This isn't a novel; you want to start with the spoiler to quickly assess the relevance of the study.
• Think critically about what you read and seek to build your own arguments; not everything may be entirely valid, examined effectively, or thoroughly investigated.
• Look up the definitions of unfamiliar words, concepts, or terminology. A good scholarly source is Credo Reference .

Taking notes as you read will save time when you go back to examine your sources. Here are some suggestions:

• Mark or highlight important text as you read [e.g., you can use the highlight text  feature in a PDF document]
• Take notes in the margins [e.g., Adobe Reader offers pop-up sticky notes].
• Highlight important quotations; consider using different colors to differentiate between quotes and other types of important text.
• Summarize key points about the study at the end of the paper. To save time, these can be in the form of a concise bulleted list of statements [e.g., intro has provides historical background; lit review has important sources; good conclusions].

Write down thoughts that come to mind that may help clarify your understanding of the research problem. Here are some examples of questions to ask yourself:

• Do I understand all of the terminology and key concepts?
• Do I understand the parts of this study most relevant to my topic?
• What specific problem does the research address and why is it important?
• Are there any issues or perspectives the author(s) did not consider?
• Do I have any reason to question the validity or reliability of this research?
• How do the findings relate to my research interests and to other works which I have read?

Adapted from text originally created by Holly Burt, Behavioral Sciences Librarian, USC Libraries, April 2018.

Another Reading Tip

When is it Important to Read the Entire Article or Research Paper

Laubepin argues, "Very few articles in a field are so important that every word needs to be read carefully." However, this implies that some studies are worth reading carefully. As painful and time-consuming as it may seem, there are valid reasons for reading a study in its entirety from beginning to end. Here are some examples:

• Studies Published Very Recently .  The author(s) of a recent, well written study will provide a survey of the most important or impactful prior research in the literature review section. This can establish an understanding of how scholars in the past addressed the research problem. In addition, the most recently published sources will highlight what is currently known and what gaps in understanding currently exist about a topic, usually in the form of the need for further research in the conclusion .
• Surveys of the Research Problem .  Some papers provide a comprehensive analytical overview of the research problem. Reading this type of study can help you understand underlying issues and discover why scholars have chosen to investigate the topic. This is particularly important if the study was published very recently because the author(s) should cite all or most of the key prior research on the topic. Note that, if it is a long-standing problem, there may be studies that specifically review the literature to identify gaps that remain. These studies often include the word review in their title [e.g., Hügel, Stephan, and Anna R. Davies. "Public Participation, Engagement, and Climate Change Adaptation: A Review of the Research Literature." Wiley Interdisciplinary Reviews: Climate Change 11 (July-August 2020): https://doi.org/10.1002/ wcc.645].
• Highly Cited .  If you keep coming across the same citation to a study while you are reviewing the literature, this implies it was foundational in establishing an understanding of the research problem or the study had a significant impact within the literature [positive or negative]. Carefully reading a highly cited source can help you understand how the topic emerged and motivated scholars to further investigate the problem. It also could be a study you need to cite as foundational in your own paper to demonstrate to the reader that you understand the roots of the problem.
• Historical Overview .  Knowing the historical background of a research problem may not be the focus of your analysis. Nevertheless, carefully reading a study that provides a thorough description and analysis of the history behind an event, issue, or phenomenon can add important context to understanding the topic and what aspect of the problem you may want to examine further.
• Innovative Methodological Design .  Some studies are significant and worth reading in their entirety because the author(s) designed a unique or innovative approach to researching the problem. This may justify reading the entire study because it can motivate you to think creatively about pursuing an alternative or non-traditional approach to examining your topic of interest. These types of studies are generally easy to identify because they are often cited in others works because of their unique approach to studying the research problem.
• Cross-disciplinary Approach .  R eviewing studies produced outside of your discipline is an essential component of investigating research problems in the social and behavioral sciences. Consider reading a study that was conducted by author(s) based in a different discipline [e.g., an anthropologist studying political cultures; a study of hiring practices in companies published in a sociology journal]. This approach can generate a new understanding or a unique perspective about the topic . If you are not sure how to search for studies published in a discipline outside of your major or of the course you are taking, contact a librarian for assistance.

Laubepin, Frederique. How to Read (and Understand) a Social Science Journal Article . Inter-University Consortium for Political and Social Research (ISPSR), 2013; Shon, Phillip Chong Ho. How to Read Journal Articles in the Social Sciences: A Very Practical Guide for Students . 2nd edition. Thousand Oaks, CA: Sage, 2015; Lockhart, Tara, and Mary Soliday. "The Critical Place of Reading in Writing Transfer (and Beyond): A Report of Student Experiences." Pedagogy 16 (2016): 23-37; Maguire, Moira, Ann Everitt Reynolds, and Brid Delahunt. "Reading to Be: The Role of Academic Reading in Emergent Academic and Professional Student Identities." Journal of University Teaching and Learning Practice 17 (2020): 5-12.

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Research Methods

You are viewing the first edition of this textbook. a second edition is available – please visit the latest edition for updated information..

This page discusses the following topics:

Research Goals

Research method types.

Before discussing research   methods , we need to distinguish them from  methodologies  and  research skills . Methodologies, linked to literary theories, are tools and lines of investigation: sets of practices and propositions about texts and the world. Researchers using Marxist literary criticism will adopt methodologies that look to material forces like labor, ownership, and technology to understand literature and its relationship to the world. They will also seek to understand authors not as inspired geniuses but as people whose lives and work are shaped by social forces.

Example: Critical Race Theory Methodologies

Critical Race Theory may use a variety of methodologies, including

• Interest convergence: investigating whether marginalized groups only achieve progress when dominant groups benefit as well
• Intersectional theory: investigating how multiple factors of advantage and disadvantage around race, gender, ethnicity, religion, etc. operate together in complex ways
• Radical critique of the law: investigating how the law has historically been used to marginalize particular groups, such as black people, while recognizing that legal efforts are important to achieve emancipation and civil rights
• Social constructivism: investigating how race is socially constructed (rather than biologically grounded)
• Standpoint epistemology: investigating how knowledge relates to social position
• Structural determinism: investigating how structures of thought and of organizations determine social outcomes

To identify appropriate methodologies, you will need to research your chosen theory and gather what methodologies are associated with it. For the most part, we can’t assume that there are “one size fits all” methodologies.

Research skills are about how you handle materials such as library search engines, citation management programs, special collections materials, and so on.

Research methods  are about where and how you get answers to your research questions. Are you conducting interviews? Visiting archives? Doing close readings? Reviewing scholarship? You will need to choose which methods are most appropriate to use in your research and you need to gain some knowledge about how to use these methods. In other words, you need to do some research into research methods!

Your choice of research method depends on the kind of questions you are asking. For example, if you want to understand how an author progressed through several drafts to arrive at a final manuscript, you may need to do archival research. If you want to understand why a particular literary work became a bestseller, you may need to do audience research. If you want to know why a contemporary author wrote a particular work, you may need to do interviews. Usually literary research involves a combination of methods such as  archival research ,  discourse analysis , and  qualitative research  methods.

Literary research methods tend to differ from research methods in the hard sciences (such as physics and chemistry). Science research must present results that are reproducible, while literary research rarely does (though it must still present evidence for its claims). Literary research often deals with questions of meaning, social conventions, representations of lived experience, and aesthetic effects; these are questions that reward dialogue and different perspectives rather than one great experiment that settles the issue. In literary research, we might get many valuable answers even though they are quite different from one another. Also in literary research, we usually have some room to speculate about answers, but our claims have to be plausible (believable) and our argument comprehensive (meaning we don’t overlook evidence that would alter our argument significantly if it were known).

A literary researcher might select the following:

Theory: Critical Race Theory

Methodology: Social Constructivism

Method: Scholarly

Skills: Search engines, citation management

Wendy Belcher, in  Writing Your Journal Article in 12 Weeks , identifies two main approaches to understanding literary works: looking at a text by itself (associated with New Criticism ) and looking at texts as they connect to society (associated with Cultural Studies ). The goal of New Criticism is to bring the reader further into the text. The goal of Cultural Studies is to bring the reader into the network of discourses that surround and pass through the text. Other approaches, such as Ecocriticism, relate literary texts to the Sciences (as well as to the Humanities).

The New Critics, starting in the 1940s,  focused on meaning within the text itself, using a method they called “ close reading .” The text itself becomes e vidence for a particular reading. Using this approach, you should summarize the literary work briefly and q uote particularly meaningful passages, being sure to introduce quotes and then interpret them (never let them stand alone). Make connections within the work; a sk  “why” and “how” the various parts of the text relate to each other.

Cultural Studies critics see all texts  as connected to society; the critic  therefore has to connect a text to at least one political or social issue. How and why does  the text reproduce particular knowledge systems (known as discourses) and how do these knowledge systems relate to issues of power within the society? Who speaks and when? Answering these questions helps your reader understand the text in context. Cultural contexts can include the treatment of gender (Feminist, Queer), class (Marxist), nationality, race, religion, or any other area of human society.

Other approaches, such as psychoanalytic literary criticism , look at literary texts to better understand human psychology. A psychoanalytic reading can focus on a character, the author, the reader, or on society in general. Ecocriticism  look at human understandings of nature in literary texts.

We select our research methods based on the kinds of things we want to know. For example, we may be studying the relationship between literature and society, between author and text, or the status of a work in the literary canon. We may want to know about a work’s form, genre, or thematics. We may want to know about the audience’s reading and reception, or about methods for teaching literature in schools.

Below are a few research methods and their descriptions. You may need to consult with your instructor about which ones are most appropriate for your project. The first list covers methods most students use in their work. The second list covers methods more commonly used by advanced researchers. Even if you will not be using methods from this second list in your research project, you may read about these research methods in the scholarship you find.

Most commonly used undergraduate research methods:

• Scholarship Methods:  Studies the body of scholarship written about a particular author, literary work, historical period, literary movement, genre, theme, theory, or method.
• Textual Analysis Methods:  Used for close readings of literary texts, these methods also rely on literary theory and background information to support the reading.
• Biographical Methods:  Used to study the life of the author to better understand their work and times, these methods involve reading biographies and autobiographies about the author, and may also include research into private papers, correspondence, and interviews.
• Discourse Analysis Methods:  Studies language patterns to reveal ideology and social relations of power. This research involves the study of institutions, social groups, and social movements to understand how people in various settings use language to represent the world to themselves and others. Literary works may present complex mixtures of discourses which the characters (and readers) have to navigate.
• Creative Writing Methods:  A literary re-working of another literary text, creative writing research is used to better understand a literary work by investigating its language, formal structures, composition methods, themes, and so on. For instance, a creative research project may retell a story from a minor character’s perspective to reveal an alternative reading of events. To qualify as research, a creative research project is usually combined with a piece of theoretical writing that explains and justifies the work.

Methods used more often by advanced researchers:

• Archival Methods: Usually involves trips to special collections where original papers are kept. In these archives are many unpublished materials such as diaries, letters, photographs, ledgers, and so on. These materials can offer us invaluable insight into the life of an author, the development of a literary work, or the society in which the author lived. There are at least three major archives of James Baldwin’s papers: The Smithsonian , Yale , and The New York Public Library . Descriptions of such materials are often available online, but the materials themselves are typically stored in boxes at the archive.
• Computational Methods:  Used for statistical analysis of texts such as studies of the popularity and meaning of particular words in literature over time.
• Ethnographic Methods:  Studies groups of people and their interactions with literary works, for instance in educational institutions, in reading groups (such as book clubs), and in fan networks. This approach may involve interviews and visits to places (including online communities) where people interact with literary works. Note: before you begin such work, you must have  Institutional Review Board (IRB)  approval “to protect the rights and welfare of human participants involved in research.”
• Visual Methods:  Studies the visual qualities of literary works. Some literary works, such as illuminated manuscripts, children’s literature, and graphic novels, present a complex interplay of text and image. Even works without illustrations can be studied for their use of typography, layout, and other visual features.

Regardless of the method(s) you choose, you will need to learn how to apply them to your work and how to carry them out successfully. For example, you should know that many archives do not allow you to bring pens (you can use pencils) and you may not be allowed to bring bags into the archives. You will need to keep a record of which documents you consult and their location (box number, etc.) in the archives. If you are unsure how to use a particular method, please consult a book about it. [1] Also, ask for the advice of trained researchers such as your instructor or a research librarian.

• What research method(s) will you be using for your paper? Why did you make this method selection over other methods? If you haven’t made a selection yet, which methods are you considering?
• What specific methodological approaches are you most interested in exploring in relation to the chosen literary work?
• What is your plan for researching your method(s) and its major approaches?
• What was the most important lesson you learned from this page? What point was confusing or difficult to understand?

Write your answers in a webcourse discussion page.

• Introduction to Research Methods: A Practical Guide for Anyone Undertaking a Research Project  by Catherine, Dr. Dawson
• Practical Research Methods: A User-Friendly Guide to Mastering Research Techniques and Projects  by Catherine Dawson
• Qualitative Inquiry and Research Design: Choosing Among Five Approaches  by John W. Creswell  Cheryl N. Poth
• Qualitative Research Evaluation Methods: Integrating Theory and Practice  by Michael Quinn Patton
• Research Design: Qualitative, Quantitative, and Mixed Methods Approaches  by John W. Creswell  J. David Creswell
• Research Methodology: A Step-by-Step Guide for Beginners  by Ranjit Kumar
• Research Methodology: Methods and Techniques  by C.R. Kothari

Strategies for Conducting Literary Research Copyright © 2021 by Barry Mauer & John Venecek is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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• Published: 13 December 2023

Arts and creativity interventions for improving health and wellbeing in older adults: a systematic literature review of economic evaluation studies

• Grainne Crealey 1 ,
• Laura McQuade 2 ,
• Roger O’Sullivan 2 &
• Ciaran O’Neill 3  

BMC Public Health volume  23 , Article number:  2496 ( 2023 ) Cite this article

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As the population ages, older people account for a larger proportion of the health and social care budget. A significant body of evidence suggests that arts and creativity interventions can improve the physical, mental and social wellbeing of older adults, however the value and/or cost-effectiveness of such interventions remains unclear.

We systematically reviewed the economic evidence relating to such interventions, reporting our findings according to PRISMA guidelines. We searched bibliographic databases (MEDLINE, EMBASE, Econlit and Web of Science and NHSEED), trial registries and grey literature. No language or temporal restrictions were applied. Two screening rounds were conducted independently by health economists experienced in systematic literature review. Methodological quality was assessed, and key information extracted and tabulated to provide an overview of the published literature. A narrative synthesis without meta-analysis was conducted.

Only six studies were identified which provided evidence relating to the value or cost-effectiveness of arts and creativity interventions to improve health and wellbeing in older adults. The evidence which was identified was encouraging, with five out of the six studies reporting an acceptable probability of cost-effectiveness or positive return on investment (ranging from £1.20 to over £8 for every £1 of expenditure). However, considerable heterogeneity was observed with respect to study participants, design, and outcomes assessed. Of particular concern were potential biases inherent in social value analyses.

Conclusions

Despite many studies reporting positive health and wellbeing benefits of arts and creativity interventions in this population, we found meagre evidence on their value or cost-effectiveness. Such evidence is costly and time-consuming to generate, but essential if innovative non-pharmacological interventions are to be introduced to minimise the burden of illness in this population and ensure efficient use of public funds. The findings from this review suggests that capturing data on the value and/or cost-effectiveness of such interventions should be prioritised; furthermore, research effort should be directed to developing evaluative methods which move beyond the confines of current health technology assessment frameworks, to capture a broader picture of ‘value’ more applicable to arts and creativity interventions and public health interventions more generally.

PROSPERO registration

CRD42021267944 (14/07/2021).

Peer Review reports

The number and proportion of older adults in the population has increased in virtually every country in the world over past decades [ 1 ]. In 2015, there were around 901 million people aged 60 years and over worldwide, by 2030, this will have increased to 1.4 billion [ 2 ]. An ageing population is one of the greatest successes of public health but it has implications for economies in numerous ways: slower labour force growth; working-age people will have to make greater provisions in welfare payments for older people who are no longer economically active; provisions for increased long-term care; and, society must adjust to the changing needs, expectations and capabilities of an expanding group of its citizens.

The Covid-19 pandemic shone an uncompromising light on the health and social care sector, highlighting the seriousness of gaps in policies, systems and services. It also focused attention on the physical and mental health consequences of loneliness and social isolation. To foster healthy ageing and improve the lives of older people, their families and communities, sustained and equitable investment in health and wellbeing is required [ 3 ]. The prevailing model of health and social care which is based ostensibly on formal care provision is unlikely to be sustainable over the longer term. New models, which promote healthy ageing and recognise the need for increasing reliance on self-care are required, as will be evidence of their effectiveness, cost-effectiveness and scalability.

Arts and creativity interventions (ACIs) can have positive effects on health and well-being, as several reviews have shown [ 4 , 5 ]. For older people, ACI’s can enhance wellbeing [ 6 , 7 , 8 , 9 ], quality of life [ 10 , 11 ] and cognitive function [ 12 , 13 , 14 , 15 , 16 ]. They can also foster social cohesion [ 17 , 18 , 19 ] and reduce social disparities and injustices [ 20 ]; promote healthy behaviour; prevent ill health (including enhancing well-being and mental health) [ 21 , 22 , 23 , 24 , 25 ], reducing cognitive decline [ 26 , 27 ], frailty [ 28 , 29 , 30 , 31 , 32 , 33 ] and premature mortality [ 34 , 35 , 36 , 37 , 38 ]); support people with stroke [ 39 , 40 , 41 , 42 ]; degenerative neurological disorders and dementias and support end of life care [ 43 , 44 ]. Moreover, ACIs can benefit not only individuals, but also others, such as supporting the well-being of formal and informal carers, enriching our knowledge of health, and improving clinical skills [ 4 , 5 ].

The benefits of ACIs have also been acknowledged at a governmental level by those responsible for delivering health and care services: The UK All-Party Parliamentary Special Interest group on Arts, Health and Wellbeing produced a comprehensive review of creative intervention for health and wellbeing [ 45 ]. This report contained three key messages: that the arts can keep us well, aid recovery and support longer better lived lives; they can help meet major challenges facing health and social care; and that the arts can save money for the health service and social care.

Despite robust scientific evidence and governmental support, no systematic literature review has collated the evidence with respect to the value, cost or cost-effectiveness of such interventions. Our objective was to assess the economic impact of ACIs aimed at improving the health and wellbeing of older adults; to determine the range and quality of available studies; identify gaps in the evidence-base; and guide future research, practice and policy.

A protocol for this review was registered at PROSPERO, an international prospective register of systematic reviews (Registration ID CRD42021267944). We used pre-determined criteria for considering studies to include in the review, in terms of types of studies, participant and intervention characteristics.

The review followed the five-step approach on how to prepare a Systematic Review of Economic Evaluations (SR-EE) for informing evidence-based healthcare decisions [ 46 , 47 , 48 ]. Subsequent to developing and registering the protocol, the International Society for Pharmacoeconomic Outcomes and Research (ISPOR) published a good practice task force report for the critical appraisal of systematic reviews with costs and cost-effectiveness outcomes (SR-CCEOs) [ 49 ]. This was also used to inform the conduct of this review.

Eligibility criteria

Full economic evaluations are regarded as the optimal type of evidence for inclusion in a SR-EE [ 46 ], hence cost-minimisation analyses (CMA), cost-effectiveness analyses (CEA), cost-utility analyses (CUA) and cost–benefit analyses (CBA) were included. Social value analyses were also included as they are frequently used to inform decision-making and commissioning of services within local government. Additionally, they represent an important intermediate stage in our understanding of the costs and consequences of public health interventions, where significant challenges exist with regard to performing full evaluations [ 50 , 51 , 52 , 53 ].

Development of search strategies

The population (P), intervention (I), comparator (C) and outcomes (O) (PICO) tool provided a framework for development of the search strategy. Studies were included if participants were aged 50 years or older (or if the average age of the study population was 50 years or over). Interventions could relate to performance art (dance, singing, theatre, drama etc.), creative and visual arts (painting, sculpture, art making and design), or creative writing (writing narratives, poetry, storytelling). The intervention had to be active (for example, creating art as opposed to viewing art; playing an instrument as opposed to listening to music). The objective of the intervention had to be to improve health and wellbeing; it had to be delivered under the guidance of a professional; delivered in a group setting and delivered on more than one occasion. No restrictions were placed on the type of comparator(s) or the type of outcomes captured in the study. We deliberately limited the study to professionally led activities to provide a sharper distinction between social events where arts and creativity may occur and arts and creativity interventions per se. We set no language restriction nor a restriction on the date from which studies were reported.

Search methods

PRESS (peer-review electronic search strategies) guidelines informed the design our search strategy [ 54 , 55 ] and an information specialist adapted the search terms (outlined in Table S 1 ) for the following electronic bibliographic databases: MEDLINE, PubMed, EMBASE, Econlit and Web of Science and NHSEED. We also inspected references of all relevant studies; and searched trials registers (ClinicalTrials.gov). Search terms used included cost, return on investment, economic, arts, music, storytelling, dancing, writing and older adult as well as social return on investment (SROI). The last search was performed on 09/11/2022. As many economic evaluations of ACIs (especially SROIs) are commissioned by government bodies or charitable organisations, a search of the grey literature was undertaken.

Handling searches

A PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow chart was used to document study selection, illustrating the numbers of records retrieved and selection flow through the screening rounds [ 56 , 57 , 58 ]; all excluded records (with rationale for exclusion) were documented.

Selection of studies

Two screening rounds were conducted independently by two health economists experienced in undertaking reviews (GC, CO’N). The first round screened the title and abstract of articles based on the eligibility criteria; those selected at this stage entered a second round of full text screening with eligibility based on the inclusion and exclusion criteria. Any disagreements were discussed among the two reviewers, with access to a third reviewer available to resolve disagreements, though this proved unnecessary.

Data extraction and management

Two reviewers extracted relevant information independently using an proforma developed specifically for the purposes of this study, which included all 35 items suggested by Wijnen et al. (2016) [ 48 ]. Information was extracted in relation to the following factors: (1) general information including study title, author, year, funding source, country, setting and study design; (2) recruitment details, sample size, demographic characteristics (age, gender) and baseline health data (diagnosis, comorbidities); (3) interventions, effectiveness and cost data; (4) type of economic evaluation, perspective, payer, beneficiary, time horizon, measure of benefit and scale of intervention; (5) quality assessment, strength of evidence, any other important information; (6) results; (7) analysis of uncertainty and (8) conclusions. The quality assessment/risk of bias checklists were included in the data extraction proforma, and picklists were used to enhance uniformity of responses. The data extraction form was piloted by two reviewers (GC and CON) on one paper and discussion used to ensure consistent application thereafter.

Assessment of study quality

Two reviewers (GC & CON) independently assessed study quality, with recourse to a third reviewer for resolution of differences though this proved unnecessary. Quality assessment was based on the type of economic evaluation undertaken. Full and partial trial-based economic evaluations were assessed using the CHEC-extended checklist [ 59 ]. SROI analyses were assessed using a SROI-specific quality framework developed for the purpose of systematic review [ 60 ].

Data analysis methods

Due to the small number of evaluations detected, possible sources of heterogeneity and a lack of consensus on appropriate methods for pooling cost-effectiveness estimates [ 61 ] a narrative synthesis analysis was undertaken.

Database searches returned 11,619 records; from this, 402 duplicates were removed leaving 11,214 reports. From these 113 reports were assessment against the inclusion and exclusion criteria resulting in 4 studies for inclusion in the review. Over 40 websites were searched for relevant content returning 2 further studies for inclusion. The PRISMA 2020 diagram is presented in Fig.  1 . A high sensitivity search strategy was adopted to ensure all relevant studies were identified, resulting in a large number of studies being excluded at the first stage of screening.

PRISMA 2020 flow diagram for new systematic reviews which include searches of databases, registers and other sources

A total of six studies were identified; key characteristics are presented in Table 1 . Identified studies were published between 2011 and 2020. Two studies used a health technology assessment (HTA) framework alongside clinical trials [ 62 , 63 ] to assess the cost-effectiveness of community singing interventions. Both evaluations scored highly on the CHEC-extended checklist (Table 2 ), with findings reported in line with the CHEERS (Consolidated Health Economic Estimation Reporting Standards) checklist 2022 [ 64 ].

Four further studies employed an SROI framework to assess art and/or craft interventions: two studies were published in the peer-reviewed literature [ 65 , 66 ] and a further two in the grey literature [ 67 , 68 ]. All four adhered closely to the suggested steps for performing an SROI and consequently secured high scores (Table 3 ). No quality differential was discerned between those studies published in the academic literature when compared with those from the grey literature.

Five of the studies were undertaken in the UK [ 63 , 66 , 67 , 68 , 69 ] and one in the US [ 63 ]. Four of the studies were designed for older adults with no cognitive impairment [ 62 , 63 , 67 , 68 ]; one was designed for participants with or without dementia [ 65 ], and another was specifically for older adults with dementia and their caregivers [ 66 ]. Three of the studies were delivered in a community setting [ 62 , 63 , 67 ], two in care homes [ 65 , 68 ] and one across a range of settings (hospital, community and residential) [ 66 ]. The length and duration of the ACIs varied; some lasted 1–2 h (with multiple classes available to participants) [ 65 ], whereas others were structured programmes with sessions lasting 90 min over a 14-week period [ 62 ]. The number of participants included in studies varied; the largest study contained data from 390 participants [ 63 ], whereas other studies measured engagement using numbers of care homes or housing associations included [ 67 , 68 ].

Costs were captured from a narrower perspective (i.e., the payer—health service) for those economic evaluations which followed a health technology assessment (HTA) framework [ 62 , 63 ]. Costs associated with providing the programme and health and social care utilisation costs were captured using cost diaries. Valuation of resource usage was in line with the reference case specified for each jurisdiction.

Social value analyses included in the review [ 65 , 66 , 67 , 68 ] captured a broader picture of cost; programme provision costs included were similar in nature to those identified using an HTA framework, however, the benefits captured went beyond the individual to capture costs to a wide range of stakeholders such as family members, activity co-ordinations and care home personnel. Costs were apportioned using financial proxies from a range of sources including HACT Social Value Bank [ 69 ] and market-based valuation methods.

The range of outcomes captured and valued across HTAs and SROIs was extensive: including, but not limited to, wellbeing, quality of life, physical health, cognitive functioning, communication, control over daily life choices, engagement and empowerment, social isolation, mobility, community inclusion, depressive symptoms, sadness, anxiety, loneliness, positive affect and interest in daily life. In the programmes assessed using an HTA framework, outcomes were captured using standardised and validated instruments, for both control and intervention groups across multiple time points. Statistical methods were used to assess changes in outcomes over time. Programmes assessed using SROI relied primarily on qualitative methods (such as reflective diaries and in-depth interviews) combined with routinely collected administrative data.

The evidence from the singing interventions was encouraging but not conclusive. The ‘Silver Song Club’ programme [ 62 ] reported a 64% probability of being cost-effective at a willingness-to-pay threshold of £30,000. This study was also included in the Public Health England (PHE) decision tool to support local commissioners in designing and implementing services to support older people’s healthy ageing, reporting a positive societal return on investment [ 70 ]. Evidence from the ‘Community of Voices’ trial [ 63 ] suggested that although intervention group members experienced statistically significant improvements in loneliness and interest in life compared to control participants, no significant group differences were observed for cognitive or physical outcomes or for healthcare costs.

A positive return on investment was reported by all social value analyses undertaken. The ‘Imagine Arts’ programme, reported a positive SROI of £1.20 for every £1 of expenditure [ 65 ]. A higher yield of between £3.20-£6.62 for each £1 invested was reported in the ‘Dementia and Imagination’ programme [ 66 ]. The ‘Craft Café’ programme, reported an SROI of £8.27 per £1 invested [ 68 ], and the ‘Creative Caring’ programme predicted a SROI of between £3 to £4 for every £1 spent [ 67 ]. The time period over which return on investment was calculated differed for each evaluation from less than one year to 4 years.

The primary finding from our review concerns the paucity of evidence relating to the value, cost and/or cost-effectiveness of ACIs aimed at improving health and wellbeing in this population. Despite few restrictions being applied to our search, only six studies were found which met our inclusion criteria. This is not indicative of research into ACIs in this population, as evidenced by the identification of ninety-three studies where arts and creativity interventions were found to support better health and wellbeing outcomes in another recent review [ 5 ]. An alternative explanation is that funders do not see the added value of undertaking such evaluations in this area. That is, for funders, the cost of evaluating an ACIs is likely to be deemed unjustified given the relatively small welfare loss a misallocation of resources to them might produce. While at first glance this may seem reasonable, it disadvantages ACIs in competing with other interventions for funding and arguably exposes an implicit prejudice in the treatment of interventions from which it may be difficult to extract profit in general. That is, the paucity of evidence, may reflect inherent biases within our political economy that favour the generation of marketable solutions to health issues from which value can be appropriated as profit. Pharmaceuticals are an obvious example of such solutions, where the literature is replete with examples of evaluations sponsored by pharmaceutical companies or where public funds are used to test the claims made by pharmaceutical companies in respect of the value of their products. If the potential of ACIs to improve health and well-being is to be robustly established, ACIs must effectively compete for funding with other interventions including those from pharma. This requires a larger, more robust evidence base than is currently available and investment in the creation of such an evidence base. As there is currently no ‘for-profit’ industry to generate such an evidence base, public funding of evaluations will be central to its creation.

Our second finding concerns the values reported in the meagre evidence we did find. In five of the six studies we identified, evidence indicated that ACIs targeted at older people offered value for money [ 62 , 65 , 66 , 67 , 68 ]. One study provided mixed evidence [ 63 ], however, in this study a ‘payer’ perspective was adopted when applying an HTA framework which, by virtue of the perspective adopted, excluded a range of benefits attributable to ACIs and public health interventions more generally. Among the four studies that adopted a SROI approach, estimated returns per £1 invested ranged from £1.20 to £8.27. Given the evident heterogeneity among studies in terms of context and methods, care is warranted in comparing estimates with each other or with other SROIs. Care is also required in accepting at face value the estimates reported given methodological issues that pertain to the current state of the art with respect to SROI. With these caveats in mind noted, the values reported for ACIs using the SROI approach are comparable with those from other SROI studies in other contexts including those as diverse as a first aid intervention [ 71 ], investment in urban greenways [ 72 ] and the provision of refuge services to those experiencing domestic violence [ 73 ] (a return on investment of £3.50-£4, £2.88-£5.81 and £4.94 respectively). Similarly, with respect to the study that adopted a cost-effectiveness approach, Coulton and colleagues (2015) reported a 64% probability of the intervention being cost-effective at a threshold of £30,000 [ 62 ]. Again, it is difficult to compare studies directly, but this is similar to that reported for interventions as diverse as a falls prevention initiative [ 74 ] and the treatment of depression using a collaborative approach [ 75 ] both in the UK. That the evidence base is meagre notwithstanding, there is, in other words, a prima facie case that ACIs are capable of offering value for money when targeted at older persons.

Our third finding relates to the state of the art with respect to SROIs in this area. Over the past 40 years, considerable time, effort and resources have been expended in the development of cost-effectiveness techniques in health and social care. While considerable heterogeneity can exist around their conduct, national guidance exists in many jurisdictions on the conduct of cost-effectiveness analyses (CEA) – such as the NICE reference case in the UK [ 76 ]– as well as in the reporting of these as set out in the CHEERS 2022 guidance [ 64 ]. This has helped raise the quality of published evaluations and the consistency with which they are reported. Despite the existence of a step-by-step guidance document on how to perform SROIs [ 77 ] which outlines how displacement effects, double counting, effect attribution and drop-off should be addressed, a significant body of work still remains to ensure that the methodology addresses a range of known biases in a robust manner. Where there is no comparator to the intervention being evaluated (as was the case in the SROIs reported here) it may be difficult to convince funders that the implicit incremental costs and benefits reported are indeed incremental and attributable to the intervention. Equally, where a comparator is present, greater consensus and standardisation is required regarding the identification, generation and application of, for example, financial proxies. Currently, SROI ratios combine value across a wide range of stakeholders, which is understandable if the objective is to capture all aspects of social benefit generated. This ratio, however, may not reflect the priorities and statutory responsibilities of healthcare funders. Whist all of the aforementioned issues can be addressed, investment is required to develop the SROI methodology further to more closely meet the needs of commissioning bodies.

Notwithstanding these challenges, social value analyses play a pivotal role within the procurement processes employed by government, local authorities and other non-departmental public bodies and should not be dismissed simply because the ‘burden of proof’ falls short of that required to secure remuneration within the health sector. As most SROIs are published in the grey literature, this means they often avoid peer scrutiny prior to publication and the potential quality assurance this can offer. It is noteworthy however that two of the SROIs included in this review [ 65 , 66 ] were published in the academic literature, suggesting that the academic community are engaging with this method which is to be applauded.

Moving forward, it is unlikely we will be able to meet all of the health and wellbeing needs of our ageing population solely in a primary or secondary care setting. New models of care are required, as are new models of funding to support interventions which can be delivered in non-healthcare settings. New hybrid models of evaluation will be required to provide robust economic evidence to assist in the allocation of scarce resources across health and non-healthcare settings; such evaluative frameworks must have robust theoretical underpinnings and be capable of delivering evidence from a non-clinical setting in a timely and cost-effective manner.

In the absence of a definitive evaluation framework for ACIs being currently available, we have a number of recommendations. First, and most importantly, all impact assessments should have a control group or credible counterfactual. This is currently not required when performing an SROI making it difficult to determine if all of the benefits ascribed to an intervention are in fact attributable. This recommendation is in line with the conclusion of a report by the London School of Economics [ 78 ] for the National Audit Office (NAO) which concluded that ‘any impact evaluation (and subsequent value for money calculation) requires construction of a counterfactual’. Second, a detailed technical appendix should accompany all impact assessments to allow independent review by a subject specialist. While this would assist peer review, it would allow providing greater transparency where peer review was not undertaken prior to publication. Furthermore, it would enable recalculation of SROI ratios to exclude ‘value’ attributable to stakeholders which are not relevant to a particular funder. Third, equity considerations should be addressed explicitly in all evaluations (this is currently not required in HTAs). Fourth, both costs and outcomes should be captured from a ‘broad’ perspective (adopting a ‘narrow’ healthcare perspective may underestimate the full economic impact), with non-healthcare sector costs being detailed as part of the analysis. Finally, data should be collected post-implementation to ensure that resources continue to be allocated efficiently.

As with any review, there are limitations which should be noted. A search of the grey literature was included as evaluations of applied public health interventions are not always reported in the academic literature. Systematically identifying grey literature and grey data can be problematic [ 79 , 80 , 81 , 82 , 83 ] as it is not collected, organised or stored in a consistent manner. Hence it is possible that we have not identified all relevant studies. Furthermore, as applied public health interventions can be performed in a non-healthcare setting we included SROIs in our review of economic evaluations. Current guidance on the systematic review of economic evaluations has been developed primarily for review of HTA as opposed to public health interventions and hence SROIs would be excluded, or if included would score poorly due to the inherent biases arising from no comparator or counterfactual being included.

This systematic review found that participation in group-based arts and creativity programmes was generally cost-effective and/or produced a positive return on investment whilst having a positive impact on older people’s physical, psychological, and social health and wellbeing outcomes. Unfortunately, the small number of studies identified, coupled with differences in methods used to assess economic impact hinders our ability to conclusively determine which types of art and creativity-based activities are more cost-effective or represent best value for money.

As well as the need for a greater focus on prevention of poor health as we age, new hybrid models of healthcare delivery are necessary to meet the needs of our ageing population. These models will integrate traditional medical care with other services such as home health aides (some of which may include artificial intelligence), telemedicine and social support networks. Alongside these, ACIs have the potential to provide a low cost, scalable, easily implementable and cost-effective solution to reduce the burden of illness in this age group and support healthy ageing.

Evidence on the cost-effectiveness of a range of ACIs is of utmost importance for policy and decision makers as it can both inform the development of policies that support the provision of ACIs in the context of ageing, but also identify the most cost-effective approaches for delivering such interventions. The development of hybrid models of evaluation, capable of capturing cost-effectiveness and social value, is becoming increasingly necessary as healthcare delivery for this age group moves beyond the realms of primary and secondary care and into the community. The development and refinement of such models will ensure a more comprehensive assessment of the impact of a diverse range of interventions providing a more nuanced understanding of the impact of an intervention. This will help inform decision making and ensure interventions are implemented in a cost-effective and socially beneficial manner.

Availability of data and materials

All data generated or analysed during this study are included in the published article and its supplementary information files.

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Acknowledgements

We would like to thank Ms. Louise Bradley (Information Resource Officer, Institute of Public Health) for her assistance in refining search strategies and literature search.

This study was supported by the Institute of Public Health (IPH), 200 South Circular Road, Dublin 8, Ireland, D08 NH90. This study was a collaboration between two health economists (GC, CO’N) and two members of staff from the funding organisation (LM, RO’S). Input from IPH staff was fundamental in defining the scope of work and research question, refining search terms and review and editing of the manuscript. Staff from IPH were not involved in quality assurance or review of papers included in the manuscript.

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Crealey, G., McQuade, L., O’Sullivan, R. et al. Arts and creativity interventions for improving health and wellbeing in older adults: a systematic literature review of economic evaluation studies. BMC Public Health 23 , 2496 (2023). https://doi.org/10.1186/s12889-023-17369-x

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Mechanical stimulation devices for mechanobiology studies: a market, literature, and patents review

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Significant advancements in various research and technological fields have contributed to remarkable findings on the physiological dynamics of the human body. To more closely mimic the complex physiological environment, research has moved from two-dimensional (2D) culture systems to more sophisticated three-dimensional (3D) dynamic cultures. Unlike bioreactors or microfluidic-based culture models, cells are typically seeded on polymeric substrates or incorporated into 3D constructs which are mechanically stimulated to investigate cell response to mechanical stresses, such as tensile or compressive. This review focuses on the working principles of mechanical stimulation devices currently available on the market or custom-built by research groups or protected by patents and highlights the main features still open to improvement. These are the features which could be focused on to perform, in the future, more reliable and accurate mechanobiology studies.

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Contextualization

The adult human body is composed of approximately 37 trillion cells, all synchronized to maintain equilibrium [ 1 ]. Cells reside in a complex and dynamic microenvironment containing biological, chemical, physical, and mechanical cues which often regulate cell proliferation, migration, differentiation, and function, and ultimately, may be responsible for the development of disease [ 2 ]. To study and understand how cells respond to mechanical stimuli or how host cells will behave upon the implantation of biomaterials, researchers use mechanical stimulation devices. These devices subject biomaterials to a particular mechanical stress with the aim of stimulating cells through deformation of the biomaterials. Devices which are currently commercially available offer limited customization and restricted force measurement capabilities [ 3 , 4 ]. As a consequence, a wide variety of apparatuses, with varying levels of sophistication, design, functionality, and precision, have been custom-built by research groups to meet particular needs, while others are protected by patents. This review focuses on the working principles, functionality, and main operational features of a number of mechanical stimulation devices developed over the past years, and highlights the main features still open to improvement.

The cellular microenvironment

All cells are in permanent interaction with the surrounding microenvironment, including the extracellular matrix (ECM) and neighbouring cells. That interaction is based on a combination of multiple cues, including biological and physical cues able to influence cell behaviour [ 5 , 6 ]. Cells are constantly and cyclically subjected to external forces whose type and magnitude are highly variable and dependent on location [ 7 ]. These forces are crucial from the beginning of cell life: throughout the development of embryos, and in everyday activities, in which cells experience shear stress during breathing and blood flow, or tensile and compressive stresses from skeletal muscle contraction, joint loading, and tendon/ligament stretching [ 8 , 9 , 10 , 11 ]. Their ability to sense externally imposed forces and mechanical properties of the surrounding ECM is denominated “cell mechanosensing” [ 12 ]. These signals are later converted into changes in intracellular biochemistry and gene expression, a process often referred to as “mechanotransduction” [ 13 ]. Mechanical stimuli include not only externally imposed forces (namely tensile, compressive, and shear forces), but also intrinsic cellular tensions generated by active cell contraction [ 7 ]. In fact, besides the intracellular response to dynamic modifications of the ECM, cells are also able to influence the environment, leading to a reciprocal interaction [ 7 , 11 ]. Both outside-in and inside-out pathways exist in mechanotransduction processes, and are able to trigger signalling cascades [ 14 , 15 ]. In this sense, the environment plays an important role in many cellular processes, such as cellular adhesion, migration, proliferation, differentiation, and apoptosis [ 15 ]. Given the importance of mechanical interactions in cellular behaviour, mechanobiology has emerged as a novel interdisciplinary field combining biology, mechanics, and engineering, which aims to understand how cells sense and behave in response to mechanical stimuli [ 7 , 11 , 16 , 17 ]. Over the past few decades, researchers have developed systems to control the cellular microenvironment and, while some focus on improving cell culture conditions, others aim to study the effects of a particular mechanical stimulus on cells by using unique cells or biomaterial-based constructs.

Moving from 2D to 3D models

Biological systems are organized into several levels of structural organization, becoming more complex as the length scale increases. Beginning at the micrometre scale, cells assemble to form tissues, which are organized into organs, which together form the organism, the highest level of organization, on the metre scale. Biological model systems range from simplified two-dimensional (2D) cell cultures to more complex three-dimensional (3D) cell cultures, organoids, tissue explants, and model organisms, such as the mouse [ 2 ]. As the complexity rises, the associated cost and physiological relevance increase, while experimental accessibility decreases [ 2 ].

The 2D and 3D cell culture systems discussed below are illustrated in Fig.  1 , in both static and dynamic modes. In conventional and static cell cultures, the nutrient supply is maintained by frequently changing the culture medium. Primary cells or established cell lines may be cultured as a 2D monolayer, in the case of an adherent culture, or as a cell suspension known as a “suspension culture”. Although monolayer cultures are easily manipulated, used worldwide in life-science research, and still accepted as the gold standard, cells are highly anisotropic. This is why it has been experimentally observed that even within a particular cell line, cellular responses may differ for an identical mechanical input [ 18 ]. Moreover, it has been reported that cells may lose some differentiated characteristics [ 19 ] and that conventional 2D culture models, whether in Petri dishes or culture flasks, do not replicate the dynamic in vivo 3D microenvironment [ 2 ]. As a consequence, over the past decades, researchers have focused on more complex cell culture models which include cell seeding on prefabricated scaffolds or incorporate cells into 3D scaffolds (usually hydrogels made of synthetic polymers). These materials allow cellular spatial organization into functional cell-based constructs. In order to keep up with continuously more complex and demanding research, bioreactor systems were developed for numerous applications, for example, in the context of cell biology research and regenerative medicine therapies. While the absence of media mixing or circulation in a static aqueous environment leads to limited diffusion of fluids or gases and contributes to cell-waste accumulation and nutrition depletion [ 5 , 20 ], 3D models with continuously mixed media in a dynamic culture allow homogeneous media and cell dispersion, better reproducing the in vivo spatial and biomechanical complexity [ 19 ]. Three-dimensional culture systems, particularly bioreactors, are not the scope of this review, but extensive works on this topic may be easily found in the literature [ 2 , 20 , 21 , 22 , 23 ]. Briefly, fluid-flow-induced bioreactors are designed to enhance nutrient supply to cell cultures and replicate tissue-specific conditions [ 24 ]. They are often grouped into fed-batch [ 25 ], spinner-flask [ 26 ], rotating [ 24 , 27 ], and perfusion bioreactors [ 21 , 28 ]. Despite the continuous media agitation in bioreactors, an efficient supply of gases and nutrients is not always assured and sample handling and maintenance of sterility are challenging. Because of the constraints related to bioreactor size, cost, and time consumption when running parallel multiple experiments, the reactor volumes in microfluidic systems can be reduced down to picolitres, while assuring a laminal-flow pattern [ 29 , 30 ]. In addition to the decrease in reagent consumption, the culture environment is particularly controlled, because cell shape, dimensionality, and density are tightly regulated in 10–100 μm channels [ 31 , 32 ]. In the category of micro-engineered devices, lab-on-a-chip-based devices are commonly used for point-of-care diagnostics and are characterized by easy handling and high performance of body fluid analyses [ 33 ]. Organ-on-a-chip devices can be used for culturing cells, spheroids, organoids, and tissue biopsies and, among their final applications, can be used for drug screening and development, disease modelling, and the study of human physiology, due to their capability of closely replicating the dynamic microenvironment of living organs [ 30 , 34 ]. However, the resultant fluid-flow-induced shear stresses may induce cell damage, and current devices lack automation and well-defined protocols [ 34 ]. In contrast to bioreactors, microfluidic-based culture models (or bioreactors on a chip) offer optimized culture conditions and precise control over the chemical and physical cellular environment through the integration of sensors [ 29 ].

Cell culture models may be in a static or dynamic mode. In 2D monolayer cultures, adherent cells are in contact with the culture vessel, neighbouring cells, and the culture medium. In non-adherent plates, 3D spheroids are grown in suspension, either without or with medium agitation (fed-batch bioreactor). In spinner-flask bioreactors, cell dispersion or cell-based constructs attached to a needle are in contact with a homogeneous medium due to agitation and medium perfusion. Rotating wall vessels enable cell culture mixing without an internal stirring mechanism by definition of a proper rotation speed. Perfusion bioreactors use continuous and fresh medium perfusion through cell-based constructs, provided by peristaltic pumps. Microfluidic systems are used for culturing and monitoring of both adherent and non-adherent cells. The fluid dynamics is represented by the red arrow

Two-dimensional culture models are based on a cell monolayer in which cells are forced to adapt to an artificial, flat, and rigid surface [ 27 ], and thus do not provide meaningful information regarding the real dynamics that living cells and tissues experience. Therefore, 3D culture models have greatly increased in number and sophistication, and have the capability of more closely replicating the dynamic microenvironment. They are also more experimentally tractable than model organisms. Despite the fact that bioreactors and microfluidic-based platforms are attractive devices for transporting nutrients and thus improving overall cell culture conditions [ 2 , 35 ], fluid-flow-induced shear stresses cannot be measured in this setting; thus, although this shear stress is considered an important contribution to cell metabolism, it cannot be considered as a mechanical input for mechanotransduction studies.

In the context of mechanobiology, external force can be applied through direct methods such as micropipette aspiration [ 36 , 37 , 38 ], atomic-force microscopy [ 38 , 39 , 40 , 41 ], and substrate deformation [ 32 , 42 , 43 , 44 ]. Indirect methods can also be used, in which cells are, for example, subjected to optical or magnetic fields [ 45 , 46 , 47 ]. In direct methods, a generated and controlled force is applied directly on cells, leading, in some cases, to a large global cell strain. On the other hand, indirect methods allow researchers to monitor deformation in different regions of a cell, but not to precisely control or measure the applied stress [ 32 , 47 ].

Mechanical stimulation devices have been developed to study cellular response to an externally applied mechanical stimulus or to mimic physiological dynamics to perform more reliable studies. For example, one can apply compressive or tensile forces on cells in a controlled way via biomaterials, as illustrated in Fig.  2 . Typically, these devices consist of a culture medium vessel, a specific space for the cell-based construct/substrate, and clamping parts to apply tensile or compressive loading in a controlled computed way [ 21 ]. Furthermore, some devices allow real-time monitoring by the use of chambers composed of light-transparent materials and multi-chamber configurations for parallel experiments [ 21 ]. Individual components, such as biomaterials, ECM, and soluble and mechanical cues, may be integrated in these systems to closely mimic the in vivo environment and study physiology and screen therapeutics.

Examples of mechanical stimulation devices used for mechanobiology studies. The commonly studied mechanical stresses are compressive and tensile stresses, either applied to cells incorporated inside 3D constructs (left) or cells seeded on a flexible substrate (right). The movement imposed on the biomaterials is represented by the red arrow

Emerging mechanical stimulation devices

Mechanical stimulation devices have gained interest due to their potential for replicating mechanical cues observed in the in vivo microenvironment, controlling mechanical and physical properties with precision, and in some cases, allowing simultaneous analysis [ 48 , 49 ]. Mechanical strains in a given material obtained by applying either tensile or compressive stresses in any direction and at controlled loading features, such as strain magnitude and frequency, create a mechanical strain environment around the cultured cells. The strain profile obtained by such substrate deformation may occur in one of the three different modes illustrated in Fig.  3 .

The substrate on which cells (represented by green circles) are cultured may be subjected to different strain modes: uniaxial, biaxial, or equiaxial, as a consequence of substrate movement imposed by the load

Novel and more complex devices have been developed to assure optimal cellular conditions during manipulation (e.g., application of a particular mechanical stimulus) and analyse the corresponding cellular behaviour in small-scale volumes. Some of the operational features of these mechanical stimulation devices are summarized in Table 1 .

Depending on the goal of a particular study and consequently of the experimental design, cells may be seeded directly on the substrate or incorporated in functional constructs. Scaffolds or cell-based constructs are 3D structures often used to regulate the environment of cells, and in this sense, cells can be manipulated by controlling the mechanical properties of the scaffold, such as elasticity, rigidity, and strain. Natural scaffolds are made of naturally derived materials, namely collagen, fibrin, and components of decellularized tissues, whereas fully synthetic matrices are often composed of poly(dimethylsiloxane) (PDMS) and poly(L-lactide-co-glycolide) (PLGA) [ 27 ]. One of the most common materials used in mechanobiology research is PDMS, because it is non-cytotoxic, autoclavable, and flexible. Furthermore, it has high optical transparency and low auto-fluorescence, which allows cell analysis by fluorescence and optical imaging techniques [ 7 , 50 ]. Hydrogels are biocompatible and biomimetic 3D structures which, once again, facilitate imaging because the cellular behaviour in their interior can be monitored. Moreover, hydrogels are typically used to apply mechanical stimuli on cells because they allow a uniform distribution of stresses throughout the structure [ 21 ].

One of the most important concerns in mechanobiology studies is to ensure aseptic conditions during experiments. Therefore, all parts of the mechanical stimulation device are assembled inside a laminal-flow biosafety cabinet and then the complete and mounted device along with its electronic components is placed in a typical culture incubator, allowing maximally sterile conditions and, when required, a long-term experiment. As already mentioned, confocal imaging is often preferable to upright microscopy techniques, mainly to overcome issues related to device size constraints. Therefore, the design of some devices considers the overall device dimensions to ensure that it fits in the microscope chamber, and the incorporation of a glass coverslip or other materials with similar optical properties to allow high-magnification imaging of the cultured or encapsulated cells, for example, for performing live imaging studies [ 32 , 42 , 51 , 52 ]. Other developments are intended to produce high-throughput capabilities, such as by increasing the number of wells/chambers which can be loaded simultaneously [ 51 , 53 , 54 , 55 , 56 , 57 ].

Commercially available cell-stimulation devices

In vitro mechanical stimulation devices have been developed to apply specific mechanical stimuli to biomaterials and, in this way, stimulate cells, for example, as they would be stimulated by external cues typically observed in physiological conditions. Some tension and compression devices are available on the market, and the most well-known companies are FLEXCELL International Corporation, TA Instruments, CellScale, IonOptix, BISS, and Strex. Some features of their commercially available products, including the type of mechanical stimulus and the maximum strain and frequency that can be applied, are summarized in Table 2 . Figure  4 is a graphical representation of the maximum strain/displacement and frequency provided by the device models.

Graphical arrangement of the commercially available mechanical stimulation devices according to their maximum strain/displacement and frequency

Some of the commercialized devices may be used to evaluate a variety of specimens, including cells seeded in monolayers, 3D cell-seeded constructs (e.g. hydrogels), natural tissues, or bioartificial tissue samples. Stretching devices commercialized by FLEXCELL use regulated vacuum pressure and positive air pressure to deform flexible-bottomed culture plates. Depending on the type of culture plate, equibiaxial or uniaxial tension may be applied. Flexcell FX-2000 and FX-4000 created by FLEXCELL International Corporation, and recently upgraded to FX-6000 T, were used to promote tensile loading [ 58 , 59 , 60 , 61 ].

The Flexcell FX-2000 cell-strain unit is composed of a circular silicone rubber membrane at the bottom of each well of the culture plate, in which biaxial strain is regulated by applying vacuum, promoting a multi-radial uniform stretch. One group exposed flexor tendon cells to biaxial tensile strain of 0.0075% at 1 Hz and analysed the formation and organization of the actin stress-fibre network and cell–cell adherent junctions under loading [ 58 ]. In another study, the same Flexcell unit was used to apply an equibiaxial cyclic strain of 3% at 0.25 Hz (2 s on, 2 s off) to human mesenchymal stem cells (hMSCs) for 16 days, which decreased proliferation and stimulated matrix mineralization over unstrained cells (Figs. 5 a– 5 c) [ 59 ]. Human embryonic stem cells (hESCs) were cultured on BioFlex culture plates coated with Matrigel and exposed to a biaxial 10% membrane strain for 10 cycles/min using the FX-4000 device [ 61 ]. The mechanical strain inhibited hESC differentiation, but self-renewal was promoted compared to an unstrained control [ 61 ]. Porcine valve interstitial cells and bone-marrow-derived hMSCs (bm-hMSCs) were cultured on BioFlex culture plates and exposed to a biaxial (radial and circumferential) tensile strain of 7%, 10%, 14%, and 20% respectively at 0.6 Hz for 4 days [ 62 ]. The strain magnitudes, which were homogenously distributed throughout the membrane, had an impact on collagen production [ 62 ]. More recently, the FX-6000 T Tension System was used on vascular smooth muscle cells derived from human-induced pluripotent stem cells (hiPSCs-VSMCs) [ 63 ]. Uniaxial cyclic tensile strain of 2.5% at 2.75 Hz applied for a period of 48 h enhanced the expression of VSMC and ECM markers and also the formation of phalloidin, mostly in a perpendicular direction to the tensile loading direction [ 63 ]. A Flexcell FX-4000 strain unit was used by Sumanasinghe et al. to apply an uniaxial cyclic tensile strain to bm-hMSCs seeded on linear 3D type I collagen matrices [ 60 ]. Cyclic tensile strains of 10% and 12% at 1 Hz induced osteogenic differentiation compared to unstrained controls after 1 and 2 weeks, without osteogenic supplements [ 60 ].

Cellular results of studies which made use of commercially available devices. Von Kossa staining of cell layers after 16 days in culture showed that compared to the unstrained condition ( a ), hMSCs that underwent mechanical strain imposed by Flexcell FX-2000 had greater matrix mineralization ( b ), as corroborated by measurement of matrix-deposited calcium ( c ) (reproduced from [ 59 ], Copyright 2003, with permission from Elsevier). d Sarcomere length analysis by actin (ACTN, red) and nuclei (DAPI, cyan) staining of samples: non-conditioned (NC), electrically conditioned (E), mechanically conditioned by an IonOptix C-stretch (M), and electromechanically conditioned (EM) (scale bar: 25 mm). Results for different conditioning procedures (1 Hz 3 d, 2 Hz 3 d, and 1 Hz 7 d) are shown in a boxplot ( e ) (reproduced from [ 64 ], Copyright 2017, with permission from Elsevier). Empty and non-loaded scaffolds were loaded by a TA Instruments BioDynamic device coupled with an Electroforce testing machine, then cut into cross sections and stained with Sirius red (for collagen) ( f ) and alizarin red (for calcium) ( g ). Absorbance of MTS (for cell-viability assessment), alizarin red, and Sirius red per loaded scaffold at day 20 was normalized to a paired non-loaded scaffold (mean±SD) ( h ), and showed an increase of matrix mineralization after loading (reproduced from [ 66 ], Copyright 2009, with permission from Elsevier). i Expression of gremlin-1 (Grem1) protein (green) in mouse primary chondrocytes increased 24 h after tensile stress loading (stress+) was applied with the Strex STB-140 system, compared to the unloading condition (stress−) (scale bar: 50 μm). j An acceleration of mouse osteoarthritis development after surgical induction was observed through safranin O staining and gremlin-1 immunofluorescence. Scale bars: 100 μm and 50 μm, respectively (reproduced from [ 69 ] authored by Chang et al. under the Creative Commons license CC BY 4.0). hMSCs: human mesenchymal stem cells

Commercialized by IonOptix, C-Pace EM is a multi-mode electromechanical stimulator which can be coupled to the C-Stretch system. This stimulator was used to apply electrical, mechanical, and combined electromechanical stimulation to human-induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs) cultured on fibronectin-coated PDMS [ 64 ]. A uniaxial cyclic tensile strain of 5% was applied at 1 Hz for 3 or 7 days and, with regard to electromechanical stimulation, cells were also stimulated with an electrical field of 3 V/cm and 4 ms biphasic pulse duration at the end of the mechanical stimulus hold phase to mimic the isovolumetric contraction. All three stimulus modes resulted in stress-fibre formation and sarcomeric length shortening, but upon electromechanical stimulus the transmembrane calcium current significantly decreased (Figs. 5 d and 5 e) [ 64 ].

A BioDynamic™ chamber mounted on an ELF3200 mechanical testing machine from the TA Instruments group was used in compressive loading studies [ 65 , 66 ]. Five per cent global strain was applied by cyclic compressive loading (for 2 h on day 9 and then every 5 days up to and including day 19) to hMSCs cultured in 3D polyurethane (PU) scaffolds, and was found to promote osteogenic differentiation and mineralized matrix production [ 65 ]. MLO-A5 osteoblastic cells cultured on PU open-cell foam scaffolds were exposed to a compressive strain of 5% at 1 Hz (for 2 h per day on days 5, 10, and 15 of culture), which promoted the production of mineralized matrix (Figs. 5 f– 5 h) [ 66 ]. Endothelial progenitor cells (EPCs) isolated from rat bone-marrow were seeded in demineralized bone matrix (DBM) scaffolds under cyclic compressive loading. Cell-based constructs were placed in a BioDynamic ELF5110 device, and after being subjected to 5% strain at 1 Hz for 4 h/day for 7 days, proliferation of EPCs increased [ 67 ].

Cell tensile loading systems from Strex are reported in the literature for diverse purposes, including the evaluation of cell adhesion and mechanotransduction studies. Meniscal root and horn cells were cultured on rat tail COL1-coated polydimethylsiloxane and subjected to 2 h and 4 h treatment with 5% and 10% uniaxial cyclic tensile strain at 0.5 Hz, using a STB-140 system. The density of both root and horn cells was reduced after mechanical treatment, whereas expression of the chondrocyte-associated genes SOX9 and COL2A1 was significantly enhanced [ 68 ]. Using the same loading system, mouse primary chondrocytes were seeded into silicon stretch chambers coated with fibronectin, and after 48 h were subjected to cyclic tensile loading (0.5 Hz, 10% elongation) for 30 min in a CO 2 incubator. The excessive loading accelerated osteoarthritis development by inducing gremlin-1 (Figs. 5 i and 5 j) [ 69 ]. In a different study, Murali et al. seeded hMSCs onto silicone chambers coated with COL1 and subjected them to tensile loading at 1 Hz frequency and 8% strain for 6, 24, 48, and 72 h, using the ST-140 model. They suggested that when subjected to uniaxial loading, hMSCs underwent tenogenic differentiation through activation of epithelial sodium channels [ 70 ]. Takahashi et al. seeded normal human lung fibroblasts onto silicon chambers coated with COL1, and a uniaxial sinusoidal cyclic tensile loading of 30 cycles/min was applied for 10 min using the ST-140 model. The concentrations of ATP in the supernatant were significantly elevated by 20% strain, but not by 4% strain. The researchers also visualized ATP release during cell stretch in real time, using the NS-600 W model. Following a single uniaxial tensile strain of 22% for 1 s duration, the release of ATP continued and increased in intensity [ 71 ].

Besides application of tensile or compressive stresses, all models of TA Instruments presented in Table 2 include pulsatile stimulation. For example, the BioDynamic 5170 and BioDynamic 5270 test instruments permit a flow range of 17–1760 mL/min. These devices are computer-controlled, allowing a static, cyclic, or intermittent deformation in a range of frequencies, amplitudes, and waveforms. The substrates used for culture plates of commercialized mechanical stimulation devices are typically flexible and light-transparent materials that enable phase-contrast, fluorescence, or scanning confocal microscopy analysis. The design and material choices allow, in some cases, simultaneous and real-time visualization using inverted microscopes. While these are often preferred to upright microscopes because they do not limit the total height of the device, it is mandatory to ensure that the focal length is not compromised. Despite the notorious progress on the tensile and compressive loading devices available on the market, the main motivation for research groups to design and fabricate their own devices is related to these device-associated costs. The cost of these devices ranges from thousands to tens of thousands of dollars, and the total price may increase when considering maintenance and the need to purchase additional device-specific accessories (such as culture well plates) in order to maximize the number of samples that can be tested at the same time [ 3 ]. Moreover, the macroscopic dimensions of some devices limit their throughput, and it is usually difficult or impossible to adapt to particular experiments such as, for example, using other substrate materials than polymers, which restricts the utility of these devices in a research context [ 3 , 51 , 52 , 72 ].

Custom-built cell-stimulation devices

Despite the focus on continuous innovation, mechanical stimulation systems available on the market present critical challenges. Consequently, numerous research groups have designed and developed custom-built devices to study the effects of tensile and compressive loading conditions on cellular behaviour.

Tensile loading devices

Several tensile loading devices with different designs and working principles have been developed in recent years. Table 3 lists uniaxial tensile loading studies in which devices were custom fabricated to meet research groups’ needs. The tensile loading working principles were divided into four major groups: four-point bending apparatuses, linear sliding rake systems, clamped samples connected to a tensile device, and vacuum-actuated tensile devices, as represented in Fig.  6 . The tensile loading devices designed by research groups are graphically arranged in Fig.  7 , according to the maximum strain/displacement and frequency under study.

Representation of four tensile working principles. In the four-point bending apparatus, the cell-based construct is supported by stationary supports and strain is distributed in the perpendicular plane to the applied load. However, the strain magnitude is not distributed uniformly between these horizontal planes because it increases from the central horizontal axis to the external medial and lateral faces (which are subjected to maximum tensile and compressive strains, respectively) [ 73 ]. Constructs can also be placed in cages and fixed at two opposite ends: one rigidly and the other to the rake attachment. The linear sliding rake is then controlled at the desired frequency and amplitude. Cell-based samples can also be clamped and connected to a tensile device (one end of the rectangular membrane is fixed while the other is connected to a computer-controlled movable frame). Finally, polymeric substrates may be exposed to uniaxial tensile loading with a vacuum-actuated tensile device. The movement imposed by the tensile loading device is represented by the red arrow

The majority of tensile loading devices involve cell culture on a circular flexible membrane (fixed along its periphery) or rectangular flexible membrane (fixed at opposite ends) (Fig.  7 ). Some of the results obtained by the studies mentioned in Table 3 are summarized in Fig.  8 .

Graphical arrangement of the devices designed and produced by research groups according to the maximum tensile strain/displacement studied at a particular frequency. Observation: despite the fact that Subramanian et al. [ 57 ] applied 2% uniaxial tensile strain to cells encapsulated in collagen constructs, the device used in this study may operate at a loading strain up to 12% at cyclic frequencies of 0.01–1 Hz

Results of studies performed with customized tensile loading devices. ALP staining at 24 h was higher for cells subjected to mechanical loading by a four-point bending device ( a ) compared to unstretched cells ( b ) and control cells ( c ) (reproduced from [ 43 ], Copyright 2008, with permission from Elsevier). d Scanning electron micrographs of 3D collagen constructs encapsulated with OB6, C2C12, or AC10 cells at day 3 either loaded by a linear sliding rake system or non-loaded (scale bar: 100 μm) indicated that the fibre orientation of loaded cells was parallel to the axis of load application (reproduced from [ 57 ], Copyright 2017, with permission from Wiley). e Staining of actin (green), sarcomeric z-lines (red), and nuclei (blue) of cardiomyocytes cultured on clamped samples connected to a tensile device revealed that after being subjected to 6 h of cyclic uniaxial tensile loading, the cells aligned in the direction of loading (white arrow) (reproduced from [ 3 ], Copyright 2018, with permission from ASME). f No significant differences in the average normalized total fluorescence of nuclei were found between non-stretched (control) and stretched groups, but F-actin fluorescence significantly increased with loading performed with a vacuum-actuated tensile device. The spatial distribution of per cent change between stretched and control average nuclei ( g ) and F-actin fluorescence ( h ) showed that expression of F-actin increased after tensile loading (reproduced from [ 52 ], Copyright 2018, with permission from Springer Science Business Media, LLC, part of Springer Nature)

Application of cyclic uniaxial (one direction) tensile strains sometimes leads to a heterogeneous biaxial strain profile, due to the Poisson effect. Therefore, custom-designed devices were developed to modulate equiaxial strains and generate a homogeneous strain environment [ 32 , 44 , 78 , 79 ]. Other groups focused on reproducing a more complex physiological environment by applying biaxial strains [ 80 , 81 ]. Tensile loading was also performed using piezoelectrically actuated pins of a Braille display [ 72 ]. Briefly, an elastomeric membrane of PDMS containing microwells was placed on top of an actuated pins array and deformed by the Braille pin movement. Each pin was independently computer-controlled and responsible for applying a cyclically radial strain (maximal 20%–25% radial and 12% tangential). Mouse myogenic C2C12 cells and human dermal microvascular endothelial cells (HDMECs) aligned to the loading direction at increasing frequencies of 0.2, 1, and 5 Hz, after 2, 4, and 12 h, in contrast to human lung adenocarcinoma epithelial A549 cells, which did not respond to tensile loading (Fig.  9 ) [ 72 ].

Schematic representation of the cross section of cells (represented in green) cultured on flexible membranes ( a ) and the deformation of the latter as the pin moves upwards ( b ). The alignment of C2C12 and HDMECs in response to loading is illustrated by fluorescent images of HDMECs ( c , d ), C2C12 myoblast cells ( e , f ), and A549 alveolar epithelial cells ( g , h ) stained with Calcein AM before loading (left column) and after being subjected to cyclical tensile loading (5 Hz for 12 h) (right column) (reproduced from [ 72 ], Copyright 2008, with permission from Elsevier). HDMECs: human dermal microvascular endothelial cells

Compressive loading devices

Microfabricated devices have been developed to apply compressive strain to cell-encapsulated constructs. Typically, the compression is achieved by loading pistons actuated by a pneumatic system. This type of compressive device can generate three types of compression: unconfined, semi-confined, and confined, all of which are illustrated in Fig.  10 .

Illustration of compression models: unconfined compression, confined compression, and semi-confined compression. The three-dimensional cell-based construct is compressed by the movement of a piston, in this case as a result of pressurized air (represented by the red arrow)

Unconfined compression was studied in the majority of compressive loading studies, as it represents the simplest microfabrication technique. One device was designed to study the influence of dynamic 10% compressive strain at 1 Hz for up to 3 weeks on chondrogenesis of goat bm-MSCs encapsulated in poly(ethylene glycol) diacrylate (PEGDA) hydrogels, as well as human embryonic body-derived (hEBd) cells encapsulated in tyrosine-glycine-aspartate-serine (YRGDS)-PEG-acrylate hydrogels [ 82 ]. The expression and synthesis of chondrocyte-specific matrix molecules were also studied, under the same loading conditions, with bovine bm-stromal cells encapsulated in agarose gels for 8 and 16 days [ 83 ]. Another group investigated the role of cyclical unconfined compression on osteogenesis by applying 10% and 20% compressive strains at 0.5 Hz for 4 h on rat pre-osteoblasts seeded into electrospun polycaprolactone (PCL) scaffolds [ 84 ]. Only the 10%-magnitude strain induced expression of osteogenic-related proteins and transcription factors, showing that elevated magnitudes may inhibit bone formation [ 84 ]. Ravichandran et al. designed a custom-fabricated device to apply a range of compressive strains to four independent chambers. hMSCs were seeded on polycaprolactone-β tricalcium phosphate (PCL-TCP) scaffolds using fibrin gel and then exposed to 0.22%, 0.88%, and 1.1% compressive strain at 1 Hz, 4 h/day for 4 weeks [ 85 ]. Cyclic physiological compression of 0.22% resulted in higher ALP activity compared to supra-physiological strains; it also up-regulated osteogenic markers and generated high mineralization levels [ 85 ]. C3H10T1/2 mouse MSCs were encapsulated in PEG hydrogels and exposed to 6%, 11%, 14%, and 26% compressive strains. Regardless of the strain magnitude, there was no significant difference in nuclear deformation, whereas cellular deformation only changed significantly at the highest strain levels [ 86 ]. The device proposed by Moraes et al. [ 86 ] consisted on an array of loading posts suspended over actuation cavities and was adopted and altered by Lee et al. to subject alginate-chondrocyte constructs to compressive strain [ 87 ]. These constructs were placed on PDMS balloons with different diameters and by varying only the cavity diameter (with the applied pressure remaining the same), it was possible to create a range of compressive strains. Because the balloons were inflated with pressurized air, compression on constructs was studied either in a static (14 kPa, 1 h) or dynamic (14 kPa, 1 Hz, 1 h) mode. Lee et al. found that the mean strain of chondrocytes was approximately 50% of the gel strain, with a permanent deformation of 9%–30% for static compression, and 0.5%–6% for dynamic compression. Finally, cell viability was found to be higher in dynamically loaded constructs, perhaps due to better nutrient transport [ 87 ]. Despite these findings, unconfined compression generates a heterogeneous strain distribution within the 3D construct. In contrast, confined compression generates a uniform strain, but challenges arise concerning its microfabrication. Semi-confined compression offers easier microfabrication, but uniform strains are only achieved in the central region of the biomaterial. These findings were obtained from the finite-element simulations (represented in Fig.  11 ), and the obtained strain fields for the three compression modes were compared [ 88 ].

Finite-element simulations of three compression modes all involved application of 10% compressive strain on PEG hydrogel (adapted from [ 88 ], Copyright 2011, with permission from IOP Publishing Ltd). Strain field within the hydrogel was generated for unconfined ( a ), confined ( b ), and semi-confined ( c ) compression modes and the radial, circumferential, and axial strains (mean and standard deviation) were plotted for the total axial thickness of the hydrogel

The same group fabricated a semi-confined compression device, which was used to generate nominal strains of 20%, 30%, 40%, and 45% on PEG and collagen hydrogels. The authors concluded that this compression model enables the study of cellular responses to precisely applied strains on a range of polymerizable biomaterials, improving the applicability and versatility of the device [ 88 ]. Zhang et al. [ 89 ] used a custom-designed dynamic-compression loading system with a stepper motor to apply a 10% compressive strain on PCL scaffolds encapsulated with hMSCs. PCL-based constructs subjected to both mechanical and biochemical stimulation supported a chondroprotective effect [ 89 ].

Review of patents

Given the promising outcomes from engineered mechanical stimulation devices in mechanobiology studies like those described above, many patent applications have been filed in the past few decades. We looked at both granted patents and patent applications, and conducted our research through the Derwent Innovation Index [ 90 ] and Google Patents [ 91 ] databases, which include European, USA, and World Patents (WIPO—World Intellectual Property Organization). The keywords were related to mechanical cell stimulation and resulted in a total of 369 records. We excluded inventions whose descriptions could not be automatically translated to English, those with unclear abstracts, and those not within the scope of this review.

A total of 44 patents of devices able to create different stimuli (tensile, compressive, shear stresses, and vibration) were comprehensively studied and selected, taking into account their in vitro cellular purposes. Table 4 summarizes the selected inventions. They are organized first by frequency level (low to moderate vs. high frequency), then by actuator type, as schematically illustrated in Fig.  12 , and finally by the imposed stress state. Inventions that obey non-conventional actuation principles are in a separate category.

Actuation principles of various mechanical cell-stimulation devices, divided by frequency level: low to high for values up to 200 Hz and extremely high frequency for values up to 100 MHz

To better organize the resulting inventions, we grouped them according to their actuation principle. From low to high frequencies, the actuation may be pneumatic (either positive or negative pressure), motor-driven, or magnetic. However, some inventors disclosed more than one functionality principle. The invention of Sittampalam et al. [ 96 ] is a drug-screening device and system that attempts to impart strain to cells in a similar manner to physiological motion and rest. According to their description, different mechanical drive systems can be adopted to move the pins and thus exert mechanical strain on cells, including a linear actuator (Fig.  13 a), an electromagnetic system, or a pneumatic system (Fig.  13 b).

The device of invention number US 2013/0059324 AI may be operated ( a ) by a mechanical drive mechanism, in which the rotation of a non-circular shaft (218) moves the movable support (216), thereby pushing the push members (214) in respect to the pins (118); or ( b ) by a pressurized mechanism (220), in which the change of fluid volume moves the support (216), imparting mechanical strain to the cells located in the wells (112). Illustrations from [ 96 ]

The device invented by Shapiro et al. [ 118 ], defined in Table 3 as belonging to the motor-driven category has, in fact, more than one approach to achieving membrane mechanical displacement (Fig.  14 ). The elastic membrane upon which the cell culture is placed is securely held in place and moved by a displacement applicator located, for example, on the bottom surface. Then, it is cyclically moved upward and downward by a force generator, deforming the membrane. This membrane deformation imparts biaxial forces (either tensile or compressive) to the cells mounted thereon, and the strain profile may be either uniform or non-uniform. The electric motor drives an actuating apparatus, such as a cam (Fig.  14 a), which revolves eccentrically about an axis, contacting the bottom surface of the rod and forcing the rod and displacement applicator to move upward to contact and deform the membrane. At the end of the upward stroke, the applicator moves to a lowered position out of contact with the membrane. The invention may also correspond to a mechanical actuated tensile apparatus which applies biaxial strain (Fig.  14 b) or simultaneously applies tensile loading to several cell cultures (Fig.  14 c). The removable and disposable wells are mounted above each available displacement applicator.

Illustrations of invention number US 5,348,879 of Shapiro et al., with three different displacement applicators (represented in the drawing by 24). In ( a ), a cam deforms the membrane, in ( b ), the membrane is secured by plates and is deformed by the displacement applicator moving upward, and in ( c ), each well is mounted above the displacement applicator. Schematics from [ 118 ]

Substrate deformation through the pneumatic actuation principle may be achieved by pressure [ 92 , 98 ] or vacuum [ 93 , 94 , 95 , 96 , 97 ], and by pulling a portion of the flexible membrane upward or downward, respectively. Despite the simple set-up, in both modes the stimulus frequency is low and may not even be capable of creating dynamic cycles. However, vacuum pumps are slower than pressure inlet. Other inventions aim to deform the membrane through pressurization of the fluid (e.g., culture medium) [ 99 , 102 ], or by creating hydrostatic pressure using a piston [ 100 ]. If not carefully designed, moving the flexible membrane upward through a displacement applicator, by pressure or manually (e.g. with a piston), may cause friction between the membrane of interest and the loading post of the device, as represented in Fig.  15 .

The displacement applicator moves up and down, which may create friction between the flexible membrane and the loading post

Hydraulic actuators are comprised of a hollow cylinder with a piston, and due to unbalanced pressure applied to the piston, a force is generated that deforms the membrane equiaxially. Although these actuators are often limited in terms of frequency, invention number DE102009057698A1 of Kiesow et al. [ 98 ] allows cyclic testing in a frequency range of 0.001 to 200 Hz.

Still, either in low- or high-frequency mode, the culture liquid in contact with the cells may be pressurized and the generated hydrostatic pressure then exerts a compressive mechanical force on cells.

The motor-driven mechanism involves the conversion of a rotary motion of an electric motor into linear displacement. The selected motor may be a stepper motor [ 103 , 104 , 106 , 107 , 108 , 109 , 110 , 111 , 117 ], a linear DC motor [ 105 , 113 , 114 , 119 , 128 ], a servo motor [ 126 ], or a voice-coil motor [ 105 , 115 ]. Stepper motors, which are composed of multiple toothed electromagnets, were preferred for some of these inventions. Despite the possibility of miniaturization, precise rotation (ranging from step angles through full 360° rotation), easy set-up and control, this type of motor is frequently slow due to a low transmission ratio (from rotational to linear movement). Moreover, it may cause small vibrations, which will act as an external disturbance to the system and thus should be considered separately from the substrate’s deformation. Linear DC motors are two-wire continuous rotation motors with each pulse being so fast that the motor seems to be rotating constantly with no stuttering. Servo motors are also fast, with high torque, and can be precisely controlled because of their accurate rotation within a limited angle. This actuation principle is particularly suitable for uniaxial tensile loading and for larger engineered constructs. In addition, it allows multiple loading modes and precise control over the strain features, such as amplitude and frequency. On the other hand, some potential disadvantages are the risk of contamination and limitation of high-throughput capabilities [ 134 ].

The linear actuation principle may be preferable because it allows for automation, requires less maintenance, and offers a broader range of strain magnitudes, frequencies, and durations of mechanical stimulus. Invention number US10,421,955B2, assigned to IonOptix LLC in 2019 (Fig.  16 ), applies a tensile strain up to 50% (at 0.01–10 Hz) on a deformable rectangular culture dish made, for example, from silicone rubber [ 111 ]. A plurality of viewing ports are provided to enable cell culture observation by microscope and exchange of the culture medium, permitting long-term experiments to be performed.

Illustrations of invention number US10,421,955B2, assigned to IonOptix LLC. ( a ) is a side view of the electromechanical stage (28a) composed by a fixed support (32) and a moving support (36) connected to a stepper motor (42). ( b ) is a perspective view illustrating six culture dishes and supporting parts of the electromechanical stage. Figures from [ 111 ]

Muthiah et al. [ 115 ] developed a mechanical tensile device, invention number US 2012/0219981Al, with dimensions of 408 mm×150 mm (much larger than a 24-well plate). It is composed of two engagement areas located at opposite ends of a flexible substrate; each one connected to a movable element and a motor to promote opposite movements, as shown in Fig.  17 . The device may include a temperature-control unit, such as a heating unit and/or a fan, to assure uniform distribution of heat and to maintain the temperature, a humidity reservoir unit, and a gas-control unit to control the gas parameters and supply. During or after tensile loading, the materials may also be imaged using a microscope base plate.

Schematics of invention number US 2012/0219981Al, assigned to Muthiah and Lane [ 115 ], in which each motor (denoted by 504 and 506) is connected to an opposite end of the cell culture device (502). The embodiment includes a water reservoir (512), an electric heating unit (508) and cooling fans (510), and computer interface connections (514). Figures from [ 115 ]

The magnetic actuation principle is an indirect method used to apply mechanical stresses on the substrate of interest. The actuating part is never in contact with the moveable one, which decreases the risk of contamination, but it is hard to control the strain and velocity. In addition, there is a potential unwanted magnetic effect on cells.

Looking further at low- to high-frequency range, some inventions apply fluid-flow-induced shear stress to cells. Despite the fact that this stimulus cannot be considered a mechanical cue for mechanotransduction studies given that it cannot be measured, Jiang et al.’s invention number CN10314657A also provides, according to the inventors, tensile stresses on cells [ 127 ], and Boronyak et al.’s invention number US 8,852,923 B2 may be adapted to impose cyclic flexural and/or tensile stresses [ 128 ].

The aforementioned inventions apply tensile, compressive, or shear stresses to cells, and some allow simultaneous electric stimulation. When cells are stimulated by deformation of the substrate on which they are cultured, those forces generate uniaxial, biaxial, or equiaxial strains (previously described in Fig.  3 ). In-plane substrate distension through frictionless platen displacement (by vacuum, pressure, or another means) creates biaxial strain traction on a flexible culture membrane and produces a uniform strain field [ 119 ]. This strain-profile output may be required for more accurate and controlled mechanotransduction studies, given that all cells cultured on the substrate are subjected to the same strain.

Actuation using piezoelectric or ultrasound elements was used to create vibration/oscillation. The main advantage is easy set-up, but the deformation created exhibits low amplitude and high frequency. In addition, the deformation of the piezo elements is not equal to the deformation of the substrate, and consequently of the cells, because losses are always present and, as the whole device vibrates, the vibrations can be even more attenuated.

Potential and future perspectives

Significant progress in research and technological fields has contributed to remarkable findings on the physiological dynamics of the human body. Basic life science research has moved from 2D culture systems to more complex 3D dynamic cultures, not only to improve cell culture conditions by promoting nutrient and oxygen flow to cells, but also to more closely mimic the complex physiological environment. Unlike bioreactors or microfluidic-based culture models, for the purpose of mechanobiology studies, cells are usually seeded on polymeric substrates or incorporated into 3D constructs and stimulated in mechanical force devices in order to investigate cell adaptation to different mechanical stresses, such as tensile or compressive stresses.

Regenerative medicine strategies involve the use of biomaterials whose mechanical properties and behaviour upon implantation may be studied in vitro by closely mimicking physiological conditions of, for example, bone [ 43 , 74 , 75 , 76 ], heart muscle [ 42 , 135 ], tendon [ 56 ], and lung [ 77 ]. By reproducing the physiological conditions to which the implants or biomaterials would be subjected, the need to perform in vivo animal testing would decrease. This would be in line with the European Directive 2010/63/EC which follows the “3Rs: reduction, refinement, and replacement” strategy to reinforce the importance of using alternative in vitro and in silico methods to obtain the maximal information from the intended product prior to clinical trials [ 136 , 137 , 138 ]. Despite more reliable studies conducted over the past years for multiple tissue-specific applications, there are still opportunities for further improvement. Mechanical stimulation devices should be designed (or integrated with other systems) to allow multiple and real-time assessment and evaluation of cell behaviour and responses at a microscale. One possibility is performing real-time imaging using non-invasive imaging techniques, fluorescence, or μCT. In order to assure optimal experimental conditions, mechanical stimulation devices could be coupled and assembled with sensors for monitor-based and cell-specific parameters, such as pH, temperature, oxygen, and secretion of small molecules and proteins.

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Acknowledgements

This work was supported by FCT (Fundação para a Ciência e a Tecnologia) through the grant SFRH/BD/141056/2018, the project PTDC/EME-EME/1442/2020 and under the national support to R&D units grant, through the reference projects UIDB/04436/2020 and UIDP/04436/2020. In addition, this work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC).

Open access funding provided by FCT|FCCN (b-on).

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Conceptualization: FMF, MG, GM and FSS; Methodology: FMF; Investigation: FMF; Writing – original draft: FMF; Writing – review & editing: all authors; Funding acquisition: FMF, GM and FSS; Supervision: OC, MG, GM and FSS.

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Melo-Fonseca, F., Carvalho, O., Gasik, M. et al. Mechanical stimulation devices for mechanobiology studies: a market, literature, and patents review. Bio-des. Manuf. 6 , 340–371 (2023). https://doi.org/10.1007/s42242-023-00232-8

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Chapter 2: Getting Started in Research

Reviewing the Research Literature

Learning Objectives

• Define the research literature in psychology and give examples of sources that are part of the research literature and sources that are not.
• Describe and use several methods for finding previous research on a particular research idea or question.

Reviewing the research literature means finding, reading, and summarizing the published research relevant to your question. An empirical research report written in American Psychological Association (APA) style always includes a written literature review, but it is important to review the literature early in the research process for several reasons.

• It can help you turn a research idea into an interesting research question.
• It can tell you if a research question has already been answered.
• It can help you evaluate the interestingness of a research question.
• It can give you ideas for how to conduct your own study.
• It can tell you how your study fits into the research literature.

What Is the Research Literature?

The  research literature  in any field is all the published research in that field. The research literature in psychology is enormous—including millions of scholarly articles and books dating to the beginning of the field—and it continues to grow. Although its boundaries are somewhat fuzzy, the research literature definitely does not include self-help and other pop psychology books, dictionary and encyclopedia entries, websites, and similar sources that are intended mainly for the general public. These are considered unreliable because they are not reviewed by other researchers and are often based on little more than common sense or personal experience. Wikipedia contains much valuable information, but the fact that its authors are anonymous and may not have any formal training or expertise in that subject area, and its content continually changes makes it unsuitable as a basis of sound scientific research. For our purposes, it helps to define the research literature as consisting almost entirely of two types of sources: articles in professional journals, and scholarly books in psychology and related fields.

Professional Journals

Professional journals  are periodicals that publish original research articles. There are thousands of professional journals that publish research in psychology and related fields. They are usually published monthly or quarterly in individual issues, each of which contains several articles. The issues are organized into volumes, which usually consist of all the issues for a calendar year. Some journals are published in hard copy only, others in both hard copy and electronic form, and still others in electronic form only.

Most articles in professional journals are one of two basic types: empirical research reports and review articles.  Empirical research reports  describe one or more new empirical studies conducted by the authors. They introduce a research question, explain why it is interesting, review previous research, describe their method and results, and draw their conclusions. Review articles  summarize previously published research on a topic and usually present new ways to organize or explain the results. When a review article is devoted primarily to presenting a new theory, it is often referred to as a theoretical article .

Most professional journals in psychology undergo a process of  double-blind peer review . Researchers who want to publish their work in the journal submit a manuscript to the editor—who is generally an established researcher too—who in turn sends it to two or three experts on the topic. Each reviewer reads the manuscript, writes a critical but constructive review, and sends the review back to the editor along with his or her recommendations. The editor then decides whether to accept the article for publication, ask the authors to make changes and resubmit it for further consideration, or reject it outright. In any case, the editor forwards the reviewers’ written comments to the researchers so that they can revise their manuscript accordingly. This entire process is double-blind, as the reviewers do not know the identity of the researcher(s), and vice versa. Double-blind peer review is helpful because it ensures that the work meets basic standards of the field before it can enter the research literature. However, in order to increase transparency and accountability some newer open access journals (e.g., Frontiers in Psychology) utilize an open peer review process wherein the identities of the reviewers (which remain concealed during the peer review process) are published alongside the journal article.

Scholarly Books

Scholarly books  are books written by researchers and practitioners mainly for use by other researchers and practitioners. A  monograph  is written by a single author or a small group of authors and usually gives a coherent presentation of a topic much like an extended review article.  Edited volumes have an editor or a small group of editors who recruit many authors to write separate chapters on different aspects of the same topic. Although edited volumes can also give a coherent presentation of the topic, it is not unusual for each chapter to take a different perspective or even for the authors of different chapters to openly disagree with each other. In general, scholarly books undergo a peer review process similar to that used by professional journals.

Literature Search Strategies

Using psycinfo and other databases.

The primary method used to search the research literature involves using one or more electronic databases. These include Academic Search Premier, JSTOR, and ProQuest for all academic disciplines, ERIC for education, and PubMed for medicine and related fields. The most important for our purposes, however, is PsycINFO, which is produced by the APA. PsycINFO is so comprehensive—covering thousands of professional journals and scholarly books going back more than 100 years—that for most purposes its content is synonymous with the research literature in psychology. Like most such databases, PsycINFO is usually available through your university library.

PsycINFO consists of individual records for each article, book chapter, or book in the database. Each record includes basic publication information, an abstract or summary of the work (like the one presented at the start of this chapter), and a list of other works cited by that work. A computer interface allows entering one or more search terms and returns any records that contain those search terms. (These interfaces are provided by different vendors and therefore can look somewhat different depending on the library you use.) Each record also contains lists of keywords that describe the content of the work and also a list of index terms. The index terms are especially helpful because they are standardized. Research on differences between women and men, for example, is always indexed under “Human Sex Differences.” Research on notetaking is always indexed under the term “Learning Strategies.” If you do not know the appropriate index terms, PsycINFO includes a thesaurus that can help you find them.

Given that there are nearly four million records in PsycINFO, you may have to try a variety of search terms in different combinations and at different levels of specificity before you find what you are looking for. Imagine, for example, that you are interested in the question of whether women and men differ in terms of their ability to recall experiences from when they were very young. If you were to enter “memory for early experiences” as your search term, PsycINFO would return only six records, most of which are not particularly relevant to your question. However, if you were to enter the search term “memory,” it would return 149,777 records—far too many to look through individually. This is where the thesaurus helps. Entering “memory” into the thesaurus provides several more specific index terms—one of which is “early memories.” While searching for “early memories” among the index terms returns 1,446 records—still too many too look through individually—combining it with “human sex differences” as a second search term returns 37 articles, many of which are highly relevant to the topic.

Depending on the vendor that provides the interface to PsycINFO, you may be able to save, print, or e-mail the relevant PsycINFO records. The records might even contain links to full-text copies of the works themselves. (PsycARTICLES is a database that provides full-text access to articles in all journals published by the APA.) If not, and you want a copy of the work, you will have to find out if your library carries the journal or has the book and the hard copy on the library shelves. Be sure to ask a librarian if you need help.

Using Other Search Techniques

In addition to entering search terms into PsycINFO and other databases, there are several other techniques you can use to search the research literature. First, if you have one good article or book chapter on your topic—a recent review article is best—you can look through the reference list of that article for other relevant articles, books, and book chapters. In fact, you should do this with any relevant article or book chapter you find. You can also start with a classic article or book chapter on your topic, find its record in PsycINFO (by entering the author’s name or article’s title as a search term), and link from there to a list of other works in PsycINFO that cite that classic article. This works because other researchers working on your topic are likely to be aware of the classic article and cite it in their own work. You can also do a general Internet search using search terms related to your topic or the name of a researcher who conducts research on your topic. This might lead you directly to works that are part of the research literature (e.g., articles in open-access journals or posted on researchers’ own websites). The search engine Google Scholar is especially useful for this purpose. A general Internet search might also lead you to websites that are not part of the research literature but might provide references to works that are. Finally, you can talk to people (e.g., your instructor or other faculty members in psychology) who know something about your topic and can suggest relevant articles and book chapters.

What to Search For

When you do a literature review, you need to be selective. Not every article, book chapter, and book that relates to your research idea or question will be worth obtaining, reading, and integrating into your review. Instead, you want to focus on sources that help you do four basic things: (a) refine your research question, (b) identify appropriate research methods, (c) place your research in the context of previous research, and (d) write an effective research report. Several basic principles can help you find the most useful sources.

First, it is best to focus on recent research, keeping in mind that what counts as recent depends on the topic. For newer topics that are actively being studied, “recent” might mean published in the past year or two. For older topics that are receiving less attention right now, “recent” might mean within the past 10 years. You will get a feel for what counts as recent for your topic when you start your literature search. A good general rule, however, is to start with sources published in the past five years. The main exception to this rule would be classic articles that turn up in the reference list of nearly every other source. If other researchers think that this work is important, even though it is old, then by all means you should include it in your review.

Second, you should look for review articles on your topic because they will provide a useful overview of it—often discussing important definitions, results, theories, trends, and controversies—giving you a good sense of where your own research fits into the literature. You should also look for empirical research reports addressing your question or similar questions, which can give you ideas about how to operationally define your variables and collect your data. As a general rule, it is good to use methods that others have already used successfully unless you have good reasons not to. Finally, you should look for sources that provide information that can help you argue for the interestingness of your research question. For a study on the effects of cell phone use on driving ability, for example, you might look for information about how widespread cell phone use is, how frequent and costly motor vehicle crashes are, and so on.

How many sources are enough for your literature review? This is a difficult question because it depends on how extensively your topic has been studied and also on your own goals. One study found that across a variety of professional journals in psychology, the average number of sources cited per article was about 50 (Adair & Vohra, 2003) [1] . This gives a rough idea of what professional researchers consider to be adequate. As a student, you might be assigned a much lower minimum number of references to use, but the principles for selecting the most useful ones remain the same.

Key Takeaways

• The research literature in psychology is all the published research in psychology, consisting primarily of articles in professional journals and scholarly books.
• Early in the research process, it is important to conduct a review of the research literature on your topic to refine your research question, identify appropriate research methods, place your question in the context of other research, and prepare to write an effective research report.
• There are several strategies for finding previous research on your topic. Among the best is using PsycINFO, a computer database that catalogs millions of articles, books, and book chapters in psychology and related fields.
• Practice: Use the techniques discussed in this section to find 10 journal articles and book chapters on one of the following research ideas: memory for smells, aggressive driving, the causes of narcissistic personality disorder, the functions of the intraparietal sulcus, or prejudice against the physically handicapped.
• Watch the following video clip produced by UBCiSchool about how to read an academic paper (without losing your mind):

Video Attributions

• “ Sample PsycINFO Search on EBSCOhost ” by APA Publishing Training . Standard YouTube Licence.
• “ Using Google Scholar (CLIP) ” by clipinfolit . CC BY (Attribution)
• “ How to Read an Academic Paper ” by UBCiSchool . CC BY (Attribution)
• Adair, J. G., & Vohra, N. (2003). The explosion of knowledge, references, and citations: Psychology’s unique response to a crisis. American Psychologist, 58 , 15–23. ↵

All the published research in a particular field.

Periodicals that publish original research articles.

A type of research article which describes one or more new empirical studies conducted by the authors.

A type of research article that summarizes previously published research on a topic and usually presents new ways to organize or explain the results.

A type of review article primarily devoted to presenting a new theory.

Books written by researchers and practitioners mainly for sue by other researchers and practitioners.

Type of scholarly book written by a single author or small group of authors, coherently presents a topic much like an extended review article.

A type of scholarly book in which an editor or small group of editors recruit many authors to write separate chapters on different aspects of the same topic.

An electronic database covering thousands of professional journals and scholarly books produced by the APA.

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• Interventions to suppress puberty in adolescents experiencing gender dysphoria or incongruence: a systematic review
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• http://orcid.org/0000-0001-5898-0900 Jo Taylor ,
• Alex Mitchell ,
• Ruth Hall ,
• Claire Heathcote ,
• Trilby Langton ,
• Lorna Fraser ,
• http://orcid.org/0000-0002-0415-3536 Catherine Elizabeth Hewitt
• Department of Health Sciences , University of York , York , UK
• Correspondence to Dr Jo Taylor, Health Sciences, University of York, York, North Yorkshire, UK; dohs-gender-research{at}york.ac.uk

Background Treatment to suppress or lessen effects of puberty are outlined in clinical guidelines for adolescents experiencing gender dysphoria/incongruence. Robust evidence concerning risks and benefits is lacking and there is a need to aggregate evidence as new studies are published.

Aim To identify and synthesise studies assessing the outcomes of puberty suppression in adolescents experiencing gender dysphoria/incongruence.

Methods A systematic review and narrative synthesis. Database searches (Medline, Embase, CINAHL, PsycINFO, Web of Science) were performed in April 2022, with results assessed independently by two reviewers. An adapted version of the Newcastle-Ottawa Scale for cohort studies was used to appraise study quality. Only moderate-quality and high-quality studies were synthesised. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses reporting guidelines were used.

Results 11 cohort, 8 cross-sectional and 31 pre-post studies were included (n=50). One cross-sectional study was high quality, 25 studies were moderate quality (including 5 cohort studies) and 24 were low quality. Synthesis of moderate-quality and high-quality studies showed consistent evidence demonstrating efficacy for suppressing puberty. Height increased in multiple studies, although not in line with expected growth. Multiple studies reported reductions in bone density during treatment. Limited and/or inconsistent evidence was found in relation to gender dysphoria, psychological and psychosocial health, body satisfaction, cardiometabolic risk, cognitive development and fertility.

Conclusions There is a lack of high-quality research assessing puberty suppression in adolescents experiencing gender dysphoria/incongruence. No conclusions can be drawn about the impact on gender dysphoria, mental and psychosocial health or cognitive development. Bone health and height may be compromised during treatment. More recent studies published since April 2022 until January 2024 also support the conclusions of this review.

PROSPERO registration number CRD42021289659.

Data availability statement

Data sharing not applicable as no datasets generated and/or analysed for this study.

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See:  http://creativecommons.org/licenses/by-nc/4.0/ .

https://doi.org/10.1136/archdischild-2023-326669

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WHAT IS ALREADY KNOWN ON THIS TOPIC

Increasing numbers of children and adolescents experiencing gender dysphoria/incongruence are being referred to specialist gender services.

National and international guidelines have changed over time and outline that medications to suppress puberty can be considered for adolescents experiencing gender dysphoria/incongruence.

Several systematic reviews report a limited evidence base for these treatments, and uncertainty about the benefits, risks and long-term effects.

WHAT THIS STUDY ADDS

No high-quality studies were identified that used an appropriate study design to assess the outcomes of puberty suppression in adolescents experiencing gender dysphoria/incongruence.

There is insufficient and/or inconsistent evidence about the effects of puberty suppression on gender-related outcomes, mental and psychosocial health, cognitive development, cardiometabolic risk, and fertility.

There is consistent moderate-quality evidence, although from mainly pre-post studies, that bone density and height may be compromised during treatment.

HOW THIS STUDY MIGHT AFFECT RESEARCH, POLICY OR PRACTICE

There is a lack of high-quality evidence to support the use of puberty suppression in adolescents experiencing gender dysphoria/incongruence, and large well-designed research is needed.

Introduction

Over the last 10-15 years, increasing numbers of children and adolescents experiencing gender dysphoria/incongruence are being referred to specialist paediatric gender services. 1 2

Gender dysphoria/incongruence in childhood is associated with high rates of co-occurring mental health and psychosocial difficulties, which can affect health and well-being. 3 Clinical guidelines recommend psychosocial care to alleviate gender-related distress and any co-occurring difficulties. For pubertal adolescents, medications to suppress or lessen effects of puberty are also outlined. Gonadotropin-releasing hormone analogues (GnRH-a) are used as first-line treatment, although other drugs with anti-androgenic properties including progestins and spironolactone are used in this population. 4 5 The effects differ depending on whether they are initiated in early puberty or mid-puberty, as well as the type of intervention used, with GnRH-a suppressing puberty when started early or suspending further progression when initiated in mid-puberty, and anti-androgens instead blocking specific downstream effects of sex hormones. 4

Rationales for puberty suppression in the Dutch treatment protocol, which has informed practice internationally, were to alleviate worsening gender dysphoria, allow time for gender exploration, and pause development of secondary sex characteristics to make passing in the desired gender role easier. 6 Practice guidelines propose other indications for puberty suppression, including allowing time and/or capacity for decision-making about masculinising or feminising hormone interventions, and improving quality of life. 4 7 8

Criteria in early treatment protocols for puberty suppression specified adolescents be at least age 12 years, at Tanner stage 2 in puberty, experienced gender dysphoria in childhood which persisted and intensified during puberty and met criteria for diagnosis of gender dysphoria. 6 It was also expected that any psychosocial difficulties that could interfere with treatment were managed. 6 The World Professional Association for Transgender Health standards of care 4 and other practice guidelines 5 8 9 have broadened these criteria, for example, removing minimum age. However, other recent guidelines have taken a more cautious approach and restricted inclusion criteria in response to uncertainties in the evidence base. 7 10

Systematic reviews have consistently found mainly low-quality evidence, limited data on key outcomes or long-term follow-up. 11–16 These reviews report that while puberty suppression may offer some benefit, there are concerns about the impact on bone health, and uncertainty regarding cognitive development, psychosocial outcomes and cardiometabolic health. They conclude there is insufficient evidence to support clinical recommendations.

The proliferation of research in this area and lack of evidence to support practice means there is an ongoing need to aggregate evidence. This systematic review aims to synthesise evidence published to April 2022 that reports outcomes of puberty suppression in adolescents experiencing gender dysphoria/incongruence.

The review forms part of a linked series examining the epidemiology, care pathways, outcomes and experiences for children and adolescents experiencing gender dysphoria/incongruence and is reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. 17 The protocol was registered on PROSPERO (CRD42021289659. 18

Search strategy

A single search strategy was used to identify studies comprising two combined concepts: ‘children’, which included all terms for children and adolescents and ‘gender dysphoria’, which included associated terms such as gender-related distress and gender incongruence, and gender identity terms including transgender, gender diverse and non-binary.

MEDLINE ( online supplemental table S1 ), EMBASE and PsycINFO through OVID, CINAHL Complete through EBSCO, and Web of Science (Social Science Citation Index) were searched (13–23 May 2021 and updated on 27 April 2022).

Supplemental material

Reference lists of included studies and relevant systematic reviews were assessed for inclusion. 11–16 19 20

Inclusion criteria

The review included published research that reported outcomes of interventions used to suppress puberty for children and/or adolescents experiencing gender dysphoria/incongruence ( table 1 ).

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Inclusion and exclusion criteria

Selection process

The results of database and other searches were uploaded to Covidence 21 and screened independently by two reviewers. Full texts of potentially relevant articles were retrieved and reviewed against inclusion criteria by two reviewers independently. Disagreements were resolved through discussion and inclusion of a third reviewer.

Data extraction

Data on study characteristics, methods and reported outcomes were extracted into prepiloted data extraction templates by one reviewer and second-checked by another.

Study quality

Critical appraisal was undertaken by two reviewers independently, with consensus reached through discussion and involvement of a third reviewer where necessary.

Quality was assessed using a modified version ( online supplemental file 1 ) of the Newcastle-Ottawa Scale for cohort studies, a validated scale of eight items covering three domains: selection, comparability and outcome. 22 Scale modification included not scoring certain question(s) for cross-sectional and single-group designs, or particular outcomes; specification of key confounders to assess comparability of cohorts; guidance regarding sufficiency of follow-up and use of numerical scores for items and overall (maximum score 9 for cohorts, 8 for pre-post and cross-sectional studies with comparator). Total scores were calculated as percentages to account for different total scores (≤50% low quality, >50%–75% moderate quality, >75% high quality).

Narrative synthesis methods were used because of heterogeneity in study design, intervention, comparator, outcome and measurement. Due to high risk of bias in low-quality studies, these were excluded from the synthesis.

When synthesising results by outcome domains, care was taken to differentiate between different study designs, comparators and interventions. Where possible, potential differences in effects by birth-registered sex, treatment duration or treatment in early puberty versus late puberty were examined.

The database search yielded 28 147 records, 3181 of which were identified as potentially relevant for the linked systematic reviews and full texts reviewed. From these, 50 studies met inclusion criteria for this review ( figure 1 ).

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Study flow diagram.

Study characteristics

Studies were published from 2006 to 2022 with the majority published in 2020–2022 (n=29). Studies were conducted in the Netherlands (n=17), 23–39 the US (n=15), 40–54 the UK (n=6), 55–60 Canada (n=4), 61–64 three in Belgium 65–67 and Israel 68–70 and one in Brazil 71 and Germany 72 ( online supplemental table S2 ).

The 50 studies included 11 cohorts comparing adolescents experiencing gender dysphoria/incongruence receiving puberty suppression with a comparator, 35 39–42 45 49 50 52 56 72 8 cross-sectional with a comparator 23 33 37 47 51 53 60 71 and 31 pre-post single group studies. 24–32 34 36 38 43 44 46 48 54 55 57–59 61–70 More than half of studies (n=29) used retrospective chart review.

All but 4 studies selected adolescents experiencing gender dysphoria/incongruence from specialist gender or endocrinology services: 43 from single services (in Belgium, Israel, the Netherlands and the UK these were large regional or national services) and 3 from multiple US services. 48–50 The other four included three US studies (national survey recruiting via community settings, 53 clinical and community settings, 51 US Military Healthcare Data Repository 54 ) and a study from Brazil recruiting via Facebook. 71

Overall, studies included 10 673 participants: 9404 were adolescents experiencing gender dysphoria/incongruence (4702 received puberty suppression, 4702 did not) and 1269 other comparators. Comparator groups included adolescents or adults experiencing gender dysphoria/incongruence who had not received puberty suppression, 35 39 40 42 51–53 60 71 72 untreated adolescents not experiencing gender dysphoria/incongruence, 36 47 50 both of these comparators 23 33 37 56 or adolescents receiving treatment for a different medical reason. 41 45 49

Most studies (n=39) assessed GnRH-a. In one, some participants received GnRH-a and some (birth-registered males) spironolactone. 62 In another, GnRH-a or progestins/anti-androgens were used but numbers taking each were not reported. 40 Among the other 11 studies, 5 assessed effects of progestins (cyproterone acetate, 66 67 lynestrenol, 65 66 medroxyprogesterone 44 and levonorgestrel-releasing intrauterine system 41 ) as alternatives to GnRH-a, 41 44 65–67 1 assessed bicalutamide 46 and 5 did not specify. 43 52–54 71

Of the 50 studies, 29 reported outcomes for feminising or masculinising hormones as well as for puberty suppression, either by including a mixed sample of those receiving the two different interventions or by assessing those who progressed to hormones following puberty suppression.

The most frequently measured outcomes were puberty suppression (n=30) and physical health outcomes (n=27) ( figure 2 , online supplemental table S3 ). Gender-related outcomes and body image were measured in five and four studies, respectively. Psychological health was measured in 13 studies, psychosocial in 9 studies and cognitive/neurodevelopmental outcomes in 3 studies. Side effects were reported in six, bone health in nine, and one study measured fertility.

Outcome categories by study quality and design.

One cross-sectional study was rated high quality, 37 25 moderate quality 23 24 29–32 34–36 39 48–51 54–59 64 65 67–69 and 24 low quality. 25–28 33 38 40–47 52 53 60–63 66 70–72 Of the 11 cohort studies, which were the only studies to include a comparator and assess outcomes over time, only 5 were rated moderate quality ( figure 2 , online supplemental table S4 ). 35 39 49 50 56

In most studies, there were concerns about sample representativeness due to single site recruitment, inclusion of a selected group and/or poor reporting of the eligible population. In studies including a comparator, most did not report or control for key differences between groups and only four used matched controls. 23 33 41 47 Most studies presented results for birth-registered males and females separately or controlled for this. Few studies controlled for age or Tanner stage or co-interventions that could influence outcomes.

Overall, studies used appropriate methods to ascertain exposure and assess outcomes. Adequacy of follow-up was evident in 18 studies, with multiple studies not reporting treatment duration, including participants receiving treatment at baseline, and not aligning follow-up with treatment initiation. Missing data at follow-up/analysis or poor reporting of this affected many studies.

Four studies did not report separate outcome data for adolescents receiving puberty suppression or masculinising/feminising hormones. 39 54 60 71 Two of these were of moderate quality and not included in the synthesis, 39 54 one of which was the only study to assess fertility outcomes. 39 One moderate-quality study assessed amplitude of click-evoked otoacoustic emissions. 23 This was excluded from the synthesis on the basis of not being clinically relevant.

Synthesis of outcomes

Gender dysphoria and body satisfaction.

Two pre-post studies measured gender dysphoria and body satisfaction (with primary and secondary sex or neutral body characteristics) and reported no change before and after receiving treatment 24 55 ( table 2 ).

Gender-related, body image, psychological, psychosocial, and cognitive/neurodevelopmental outcomes

Psychological health

One cross-sectional 37 and two pre-post studies 24 55 measured symptoms of depression (n=1), anxiety (n=1), anger (n=1), internalising and externalising symptoms (n=3), suicide and/or self-harm (n=2) and psychological functioning (n=2).

Three studies assessed internalising and externalising symptoms with one reporting improvements in both (pre-post 24 ), one improvement in internalising but not externalising symptoms when compared with adolescents under assessment by a gender service (cross-sectional 37 ) and one observed no change in either (pre-post). 55

For other psychological outcomes, there was either a single study, or two studies showing inconsistent results, with studies reporting either a small to moderate significant improvement or no change ( table 2 ).

Psychosocial outcomes

One cohort 56 and two pre-post 24 55 studies measured psychosocial functioning, one pre-post study assessed quality of life 55 and one cross-sectional study measured peer-relations ( table 2 ). 37

For psychosocial functioning, both pre-post studies reported no clinically significant change at follow-up. 24 55 The cohort study compared adolescents who were not immediately eligible for puberty suppression and received psychological support only, and adolescents who additionally received GnRH-a after 6 months. 56 Improvements were seen in both groups after 6 months of psychological support. This improvement was maintained over time for those receiving psychological support only. For those receiving GnRH-a, further improvements were observed at 12 and 18 months. At 18 months, psychosocial functioning in this group was considerably higher than in those still waiting for puberty suppression, and similar to adolescents not experiencing gender dysphoria/incongruence. However, there were considerably fewer participants included at final follow-up.

There was no change in quality of life pre-post, 55 and treated adolescents had better peer-relations compared with adolescents under assessment at a gender service but poorer peer-relations than adolescents not experiencing gender dysphoria/incongruence. 37

Cognitive/neurodevelopmental outcomes

One cross-sectional study measured executive functioning and found no difference between adolescents who were treated for <1 year compared with those not treated, but worse executive functioning in those treated for >1 year compared with those not treated. 51 A pre-post study found no differences in features typically associated with autism spectrum condition after treatment ( table 2 ). 59

Physical health outcomes

Bone health.

Five studies found decreases in bone mineral apparent density and z-scores pre-post treatment; however, absolute measures generally remained stable or increased/decreased slightly. 29 32 34 55 58 Results were similar across birth-registered males and females. 29 32 55 58 One study considered timing of treatment, and found similar decreases among those starting GnRH-a in early or late puberty ( table 3 ). 32

Physical health outcomes and side effects

Cardiometabolic health

Twelve pre-post studies measured body mass index (BMI), and in 10 studies there was no evidence of a clinically significant change in BMI and/or BMI SD score. 29 30 32 34 55 57 65 67–69 In one study, BMI increased for birth-registered males but not females. 58 Another study found BMI increased for birth-registered females who started GnRH-a in early puberty or mid-puberty, and birth-registered males in early puberty. 36

Three studies assessed cholesterol markers, one after GnRH-a (no changes), 34 one after cyproterone acetate (decrease in high-density lipoprotein (HDL) and triglycerides) 67 and one after lynestrenol (decrease in HDL, increase in low-density lipoprotein). 65 Three studies assessing GnRH-a reported blood pressure: two found similar systolic and diastolic blood pressure before and after treatment, 34 68 and one found a non-clinically significant increase in diastolic but not systolic blood pressure. 69 Two studies measured markers of diabetes (fasting glucose, HbA1c and/or insulin) and noted no changes. 65 67

Other physiological parameters

Five pre-post studies assessed other parameters from blood tests undertaken at baseline and follow-up, 30 31 34 65 67 three in those treated with GnRH-a, 30 31 34 one lynestrenol 65 and one cyproterone acetate. 67 Measurements included haemoglobin count (n=3), haematocrit percentage (n=3), creatinine (n=4), aspartate aminotransferase (n=3), alanine aminotransferase (n=3), γ-glutamyl transferase (n=1), alkaline phosphatase (n=2), prolactin (n=2), free thyroxin (n=3), thyroid-stimulating hormone (n=3), sex hormone binding globulin (n=3), vitamin D levels (n=1), dehydroepiandrosterone sulfate (n=3) and androstenedione (n=2). For most outcomes, no changes were reported. Where there were changes, these were not consistent in direction across studies.

One pre-post study assessing GnRH-a reported QTc prolongation, 64 and found no change in mean QTc, with no participants outside normal range.

Side effects

A cohort study of GnRH-a reported side effects including mild headaches or hot flushes (~20%) and moderate/severe headaches or hot flushes, mild fatigue, mood swings, weight gain and sleep problems (<10%) ( table 3 ). 55

Two studies assessed other medications and reported headaches and hot flushes as common and an increase in acne in a sample of birth-registered females receiving lynestrenol, 65 and complaints of fatigue in birth-registered males receiving cyproterone acetate. 67

Puberty suppression

Hormone levels.

Hormone levels were reported in nine studies of GnRH-a (two cohort, 49 50 seven pre-post 30 34 36 48 55 68 69 ), two in birth-registered females, 34 69 one in birth-registered males 68 and six including both ( table 4 ). 30 36 48–50 55

Puberty suppression outcomes

Five studies reported decreases in luteinising hormone, follicle-stimulating hormone, oestradiol and testosterone after receiving GnRH-a. 30 34 48 68 69 Another study, which reported luteinising and follicle-stimulating hormones, also found decreases in both pre-post. 55 One study reported that where baseline levels were high due to puberty starting, decreases were reported in testosterone and oestradiol. 36 One cohort study reporting pre-post data found smaller decreases in luteinising hormone, follicle-stimulating hormone, oestradiol and testosterone compared with other studies; however, it included a younger population, some of who were likely prepubertal. 50 The other cohort study included a comparator of adolescents with precocious puberty and found similar decreases in luteinising hormone and oestradiol. 49

One pre-post study of lynestrenol (birth-registered females) found a decrease in luteinising hormones but not follicle-stimulating hormone, oestradiol or testosterone. 65 One study of cyproterone acetate (birth-registered males) found no changes in luteinising hormone, follicle-stimulating hormone or oestradiol, but a decrease in total testosterone. 67

Pubertal progression

Puberty development was reported in four studies (two cohort, two pre-post). 30 35 49 67 One only included birth-registered males, 67 and three included both birth-registered males and females. 30 35 49

A cohort study assessing GnRH-a reported clinical pubertal escape in 2/21 adolescents treated for gender dysphoria/incongruence, in the form of breast enlargement or testicular enlargement together with deepening of voice, compared with no children treated for precocious puberty. 49 A pre-post study reported a decrease in testicular volume in birth-registered males, but unclear results with regard to breast development in birth-registered females (most started treatment at Tanner stage 4–5). 30 A pre-post study of birth-registered males using cyproterone acetate reported decreases in facial shaving and spontaneous erections. 67

A cohort study assessed whether secondary sex characteristics differed depending on receipt or timing of GnRH-a, and whether this affected which surgical interventions/techniques were later used. 35 The study found breast size was smallest in birth-registered females who received GnRH-a in Tanner stage 2/3 and largest in untreated participants. Those treated early in puberty were less likely to require a mastectomy and when surgery was required it was less burdensome. In birth-registered males, penile length was greater in those who received GnRH-a at Tanner stage 4/5 compared with Tanner stage 2/3, and greatest in untreated participants. 35 Those who received GnRH-a early required more invasive vaginoplasty techniques than those who received it later or not at all.

Menstrual suppression

Three studies (one cohort, two pre-post) measured menstrual suppression in birth-registered females, and found full suppression at follow-up, 30 49 55 which was similar to the effect seen in those with precocious puberty in the cohort study. 49

Height/Growth

Eleven studies (1 cohort, 50 10 pre-post 29 30 32 34 36 55 57 58 65 67 ) reported height, nine after GnRH-a, 29 30 32 34 36 50 55 57 58 one lynestrenol 65 and one cyproterone acetate. 67 The cohort study found a similar height velocity between the GnRH-a group and adolescent controls. 50 Six studies reported height Z or SD score 29 30 34 55 57 67 with two studies finding no change, 34 55 two a decrease for birth-registered males but not females, 29 57 one a decrease across birth-registered males and females 30 and one a decrease in birth-registered males with cyproterone acetate. 67 Absolute measures of height generally increased slightly or remained the same. 29 30 32 34 36 58 65 67

Body composition

Two studies reported changes in body composition pre-post, 30 57 reporting a significant decrease in lean mass SD score 57 and percentage 30 in males and females. One also measured body fat percentage and reported significant increases in both groups. 30

Bone geometry

One pre-post study measured the subperiosteal width and endocortical diameter of the hip bone and found that in birth-registered males these increased in those starting GnRH-a in early puberty and mid-puberty, but only in the early puberty group for birth-registered females. 36

This systematic review identified 50 studies reporting outcomes relating to puberty suppression in adolescents experiencing gender dysphoria/incongruence. No high-quality studies using an appropriate design were identified, and only four measured gender dysphoria as an outcome. Only 5 of the 11 cohort studies, which were the only studies to compare groups over time, were rated as moderate quality. 35 40 49 50 56

There was evidence from multiple mainly pre-post studies that puberty suppression exerts its expected physiological effect, as previously demonstrated in children with precocious puberty. 73 In adolescents experiencing gender dysphoria/incongruence, puberty suppression is initiated at different stages of puberty, 74 and two studies found that the effects on secondary sex characteristics may vary depending on whether treatment is initiated in early puberty versus mid-puberty, with potentially different outcomes for birth-registered males and females. 30 35 Multiple studies also found that bone density is compromised during puberty suppression, and gains in height may lag behind that seen in other adolescents. High-quality research is needed to confirm these findings; however, these potential risks should be explained to adolescents considering puberty suppression.

These findings add to other systematic reviews in concluding there is insufficient and/or inconsistent evidence about the effects of puberty suppression on gender dysphoria, body satisfaction, psychological and psychosocial health, cognitive development, cardiometabolic risk and fertility. 11–16 Regarding psychological health, one recent systematic review 14 reported some evidence of benefit while others have not. The results in this review found no consistent evidence of benefit. Inclusion of only moderate-quality to high-quality studies may explain this difference, as 8 of the 12 studies reporting psychological outcomes were rated as low-quality.

The lack of representativeness of samples and comparability of selected control groups were key concerns across studies. Only one study attempted to compare puberty suppression with psychosocial care, which is the only other treatment offered for gender dysphoria/incongruence in childhood, and this included a small sample, limited analyses, and little detail about the intervention. 56 Other studies lacked information about any psychological care provided to participants, and in studies that included a comparator there was limited information about any differences between groups. Large, well-designed studies with appropriate comparators that enable long-term outcomes of puberty suppression to be measured are needed.

Many studies reported effects of both puberty suppressants and hormones used in later adolescence for feminisation/masculinisation. In adolescents, GnRH-a often continues during hormone treatment, 74 or for adolescents who do not receive puberty suppression, GnRH-a or other anti-androgens may be offered at initiation of hormones. 66 This makes long-term follow-up of puberty suppression difficult to assess, including any differences between the types of interventions that are offered and when these are initiated, and the few studies reporting long-term outcomes either did not control for this or reported overall effects for both interventions. Although recent studies suggest nearly all adolescents who receive puberty suppression go on to feminising/masculinising hormones, 74–76 research is still needed to assess whether suppression will have any lasting effects for those who do not. Aggregation of studies reporting proportions of adolescents who progress to hormones and reasons for discontinuation would also offer useful insights.

Included studies assessed different outcomes across various outcome domains and employed multiple different measures. Agreement about the primary aim and related core outcomes of puberty suppression in this population would help to ensure studies measure key outcomes and facilitate future aggregation of evidence. Expert consensus recommendations to guide the methods and domains for assessing the neurodevelopmental effects of puberty suppression have been developed 77 ; however, there is currently no agreement across other outcome domains.

Strengths and limitations

Strengths include a published protocol with robust search strategies, use of PRISMA guidelines and comprehensive synthesis of moderate and high quality studies. Poor reporting across studies may have resulted in moderate-quality studies being rated low-quality and excluded from synthesis. As searches were conducted up to April 2022, this review does not include more recently published studies. However, the findings are in line with previous reviews despite the inclusion of numerous additional studies. In an update of the National Institute for Health and Care Excellence evidence review of GnRH-a performed in April 2023, 78 nine additional studies were identified, two of which they felt might materially affect their conclusions. 72 74 One was already included in this review, 72 and the other examined treatment trajectories which was not an outcome of interest. 74

Of other studies that we are aware have been published since April 2022 until January 2024, very few used a cohort design or an appropriate comparator and were of a similar low quality to moderate quality as the studies summarised in this review. Of those likely to contribute new data for synthesis, five examined physical growth and development, 79–83 one cardiometabolic health 84 and one psychological health. 85 The latter, a study from the US, found that adolescents who received puberty suppression before assessment for masculinising or feminising hormones had fewer symptoms of depression, anxiety, stress and suicidal thoughts compared with those who had not received puberty suppression. A sensitivity analysis found similar results, although no difference in suicidal thoughts. 85 Adding this study would provide no further clarity about whether puberty suppression improves psychological health due to the inconsistency of results between studies, and the limited high-quality research measuring these outcomes.

Two studies from the Netherlands found that height growth and bone maturation both decelerated during GnRH-a treatment, 80 81 and a third assessing only bone health found the same. 83 A Belgian study found stable height growth in birth-registered females but deceleration in birth-registered males. 82 These studies add strength to the conclusion that bone health and adult height may be compromised during GnRH-a, although like in previous studies the participants went on to receive masculinising or feminising hormones, and therefore the long-term outcomes of puberty suppression alone were not possible to determine.

Another new study, also from the Netherlands, assessed changes in body composition. 79 This found that in both birth-registered males and females lean mass z-scores decreased during puberty suppression and fat mass z-scores increased, although the rate and duration of change differed by birth-registered sex. These changes were also found in the two studies synthesised, 30 57 but as all three included no comparator uncertainty continues about the effect of puberty suppression on body composition.

A large study of adults in the US examined whether receipt of hormone interventions during adolescence was associated with cardiometabolic-related diagnoses, and for GnRH-a found no statistically significant differences for any diagnosis. 84 However, the study uses a retrospective cross-sectional design and is the only study to have examined cardiometabolic diagnoses, so no conclusions can be drawn about these outcomes.

To our knowledge, there are no additional moderate-quality or high-quality studies that have measured psychosocial or fertility outcomes, and only a single study assessing cognitive effects which measured a different outcome (white matter microstructure) to those included in this review. 86

Conclusions

There are no high-quality studies using an appropriate study design that assess outcomes of puberty suppression in adolescents experiencing gender dysphoria/incongruence. No conclusions can be drawn about the effect on gender-related outcomes, psychological and psychosocial health, cognitive development or fertility. Bone health and height may be compromised during treatment. High-quality research and agreement on the core outcomes of puberty suppression are needed.

Ethics statements

Patient consent for publication.

Not applicable.

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Supplementary materials

Supplementary data.

This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

• Data supplement 1
• Data supplement 2
• Data supplement 3
• Data supplement 4
• Data supplement 5

Contributors LF, CEH, RH, TL and JT contributed to the conception of this review. RH, CEH, CH, AM and JT contributed to screening and selection. AM and JT completed data extraction. CEH, RH, AM and JT contributed to critical appraisal. CEH, AM and JT completed the synthesis and drafted the manuscript. All authors contributed to interpretation and reviewed and approved the manuscript prior to submission. CEH accepts full responsibility for the finished work and/or the conduct of the study, had access to the data, and controlled the decision to publish.

Funding This work was funded by NHS England to inform the Cass Review (Independent review of gender identity services for children and young people). The funder and Cass Review team had a role in commissioning the research programme but no role in the study conduct, interpretation or conclusion.

Competing interests None declared.

Provenance and peer review Commissioned; externally peer reviewed.

Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

Linked Articles

• Original research Clinical guidelines for children and adolescents experiencing gender dysphoria or incongruence: a systematic review of guideline quality (part 1) Jo Taylor Ruth Hall Claire Heathcote Catherine Elizabeth Hewitt Trilby Langton Lorna Fraser Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326499
• Original research Care pathways of children and adolescents referred to specialist gender services: a systematic review Jo Taylor Ruth Hall Trilby Langton Lorna Fraser Catherine Elizabeth Hewitt Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326760
• Original research Psychosocial support interventions for children and adolescents experiencing gender dysphoria or incongruence: a systematic review Claire Heathcote Jo Taylor Ruth Hall Stuart William Jarvis Trilby Langton Catherine Elizabeth Hewitt Lorna Fraser Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326347
• Original research Gender services for children and adolescents across the EU-15+ countries: an online survey Ruth Hall Jo Taylor Claire Heathcote Trilby Langton Catherine Elizabeth Hewitt Lorna Fraser Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326348
• Original research Impact of social transition in relation to gender for children and adolescents: a systematic review Ruth Hall Jo Taylor Catherine Elizabeth Hewitt Claire Heathcote Stuart William Jarvis Trilby Langton Lorna Fraser Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326112
• Original research Characteristics of children and adolescents referred to specialist gender services: a systematic review Jo Taylor Ruth Hall Trilby Langton Lorna Fraser Catherine Elizabeth Hewitt Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326681
• Original research Clinical guidelines for children and adolescents experiencing gender dysphoria or incongruence: a systematic review of recommendations (part 2) Jo Taylor Ruth Hall Claire Heathcote Catherine Elizabeth Hewitt Trilby Langton Lorna Fraser Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326500
• Original research Masculinising and feminising hormone interventions for adolescents experiencing gender dysphoria or incongruence: a systematic review Jo Taylor Alex Mitchell Ruth Hall Trilby Langton Lorna Fraser Catherine Elizabeth Hewitt Archives of Disease in Childhood 2024; - Published Online First: 09 Apr 2024. doi: 10.1136/archdischild-2023-326670
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Evidence Review of the Adverse Effects of COVID-19 Vaccination and Intramuscular Vaccine Administration

Vaccines are a public health success story, as they have prevented or lessened the effects of many infectious diseases. To address concerns around potential vaccine injuries, the Health Resources and Services Administration (HRSA) administers the Vaccine Injury Compensation Program (VICP) and the Countermeasures Injury Compensation Program (CICP), which provide compensation to those who assert that they were injured by routine vaccines or medical countermeasures, respectively. The National Academies of Sciences, Engineering, and Medicine have contributed to the scientific basis for VICP compensation decisions for decades.

HRSA asked the National Academies to convene an expert committee to review the epidemiological, clinical, and biological evidence about the relationship between COVID-19 vaccines and specific adverse events, as well as intramuscular administration of vaccines and shoulder injuries. This report outlines the committee findings and conclusions.

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StatPearls [Internet].

Qualitative study.

Steven Tenny ; Janelle M. Brannan ; Grace D. Brannan .

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Last Update: September 18, 2022 .

• Introduction

Qualitative research is a type of research that explores and provides deeper insights into real-world problems. [1] Instead of collecting numerical data points or intervene or introduce treatments just like in quantitative research, qualitative research helps generate hypotheses as well as further investigate and understand quantitative data. Qualitative research gathers participants' experiences, perceptions, and behavior. It answers the hows and whys instead of how many or how much. It could be structured as a stand-alone study, purely relying on qualitative data or it could be part of mixed-methods research that combines qualitative and quantitative data. This review introduces the readers to some basic concepts, definitions, terminology, and application of qualitative research.

Qualitative research at its core, ask open-ended questions whose answers are not easily put into numbers such as ‘how’ and ‘why’. [2] Due to the open-ended nature of the research questions at hand, qualitative research design is often not linear in the same way quantitative design is. [2] One of the strengths of qualitative research is its ability to explain processes and patterns of human behavior that can be difficult to quantify. [3] Phenomena such as experiences, attitudes, and behaviors can be difficult to accurately capture quantitatively, whereas a qualitative approach allows participants themselves to explain how, why, or what they were thinking, feeling, and experiencing at a certain time or during an event of interest. Quantifying qualitative data certainly is possible, but at its core, qualitative data is looking for themes and patterns that can be difficult to quantify and it is important to ensure that the context and narrative of qualitative work are not lost by trying to quantify something that is not meant to be quantified.

However, while qualitative research is sometimes placed in opposition to quantitative research, where they are necessarily opposites and therefore ‘compete’ against each other and the philosophical paradigms associated with each, qualitative and quantitative work are not necessarily opposites nor are they incompatible. [4] While qualitative and quantitative approaches are different, they are not necessarily opposites, and they are certainly not mutually exclusive. For instance, qualitative research can help expand and deepen understanding of data or results obtained from quantitative analysis. For example, say a quantitative analysis has determined that there is a correlation between length of stay and level of patient satisfaction, but why does this correlation exist? This dual-focus scenario shows one way in which qualitative and quantitative research could be integrated together.

Examples of Qualitative Research Approaches

Ethnography

Ethnography as a research design has its origins in social and cultural anthropology, and involves the researcher being directly immersed in the participant’s environment. [2] Through this immersion, the ethnographer can use a variety of data collection techniques with the aim of being able to produce a comprehensive account of the social phenomena that occurred during the research period. [2] That is to say, the researcher’s aim with ethnography is to immerse themselves into the research population and come out of it with accounts of actions, behaviors, events, etc. through the eyes of someone involved in the population. Direct involvement of the researcher with the target population is one benefit of ethnographic research because it can then be possible to find data that is otherwise very difficult to extract and record.

Grounded Theory

Grounded Theory is the “generation of a theoretical model through the experience of observing a study population and developing a comparative analysis of their speech and behavior.” [5] As opposed to quantitative research which is deductive and tests or verifies an existing theory, grounded theory research is inductive and therefore lends itself to research that is aiming to study social interactions or experiences. [3] [2] In essence, Grounded Theory’s goal is to explain for example how and why an event occurs or how and why people might behave a certain way. Through observing the population, a researcher using the Grounded Theory approach can then develop a theory to explain the phenomena of interest.

Phenomenology

Phenomenology is defined as the “study of the meaning of phenomena or the study of the particular”. [5] At first glance, it might seem that Grounded Theory and Phenomenology are quite similar, but upon careful examination, the differences can be seen. At its core, phenomenology looks to investigate experiences from the perspective of the individual. [2] Phenomenology is essentially looking into the ‘lived experiences’ of the participants and aims to examine how and why participants behaved a certain way, from their perspective . Herein lies one of the main differences between Grounded Theory and Phenomenology. Grounded Theory aims to develop a theory for social phenomena through an examination of various data sources whereas Phenomenology focuses on describing and explaining an event or phenomena from the perspective of those who have experienced it.

Narrative Research

One of qualitative research’s strengths lies in its ability to tell a story, often from the perspective of those directly involved in it. Reporting on qualitative research involves including details and descriptions of the setting involved and quotes from participants. This detail is called ‘thick’ or ‘rich’ description and is a strength of qualitative research. Narrative research is rife with the possibilities of ‘thick’ description as this approach weaves together a sequence of events, usually from just one or two individuals, in the hopes of creating a cohesive story, or narrative. [2] While it might seem like a waste of time to focus on such a specific, individual level, understanding one or two people’s narratives for an event or phenomenon can help to inform researchers about the influences that helped shape that narrative. The tension or conflict of differing narratives can be “opportunities for innovation”. [2]

Research Paradigm

Research paradigms are the assumptions, norms, and standards that underpin different approaches to research. Essentially, research paradigms are the ‘worldview’ that inform research. [4] It is valuable for researchers, both qualitative and quantitative, to understand what paradigm they are working within because understanding the theoretical basis of research paradigms allows researchers to understand the strengths and weaknesses of the approach being used and adjust accordingly. Different paradigms have different ontology and epistemologies . Ontology is defined as the "assumptions about the nature of reality” whereas epistemology is defined as the “assumptions about the nature of knowledge” that inform the work researchers do. [2] It is important to understand the ontological and epistemological foundations of the research paradigm researchers are working within to allow for a full understanding of the approach being used and the assumptions that underpin the approach as a whole. Further, it is crucial that researchers understand their own ontological and epistemological assumptions about the world in general because their assumptions about the world will necessarily impact how they interact with research. A discussion of the research paradigm is not complete without describing positivist, postpositivist, and constructivist philosophies.

Positivist vs Postpositivist

To further understand qualitative research, we need to discuss positivist and postpositivist frameworks. Positivism is a philosophy that the scientific method can and should be applied to social as well as natural sciences. [4] Essentially, positivist thinking insists that the social sciences should use natural science methods in its research which stems from positivist ontology that there is an objective reality that exists that is fully independent of our perception of the world as individuals. Quantitative research is rooted in positivist philosophy, which can be seen in the value it places on concepts such as causality, generalizability, and replicability.

Conversely, postpositivists argue that social reality can never be one hundred percent explained but it could be approximated. [4] Indeed, qualitative researchers have been insisting that there are “fundamental limits to the extent to which the methods and procedures of the natural sciences could be applied to the social world” and therefore postpositivist philosophy is often associated with qualitative research. [4] An example of positivist versus postpositivist values in research might be that positivist philosophies value hypothesis-testing, whereas postpositivist philosophies value the ability to formulate a substantive theory.

Constructivist

Constructivism is a subcategory of postpositivism. Most researchers invested in postpositivist research are constructivist as well, meaning they think there is no objective external reality that exists but rather that reality is constructed. Constructivism is a theoretical lens that emphasizes the dynamic nature of our world. “Constructivism contends that individuals’ views are directly influenced by their experiences, and it is these individual experiences and views that shape their perspective of reality”. [6] Essentially, Constructivist thought focuses on how ‘reality’ is not a fixed certainty and experiences, interactions, and backgrounds give people a unique view of the world. Constructivism contends, unlike in positivist views, that there is not necessarily an ‘objective’ reality we all experience. This is the ‘relativist’ ontological view that reality and the world we live in are dynamic and socially constructed. Therefore, qualitative scientific knowledge can be inductive as well as deductive.” [4]

So why is it important to understand the differences in assumptions that different philosophies and approaches to research have? Fundamentally, the assumptions underpinning the research tools a researcher selects provide an overall base for the assumptions the rest of the research will have and can even change the role of the researcher themselves. [2] For example, is the researcher an ‘objective’ observer such as in positivist quantitative work? Or is the researcher an active participant in the research itself, as in postpositivist qualitative work? Understanding the philosophical base of the research undertaken allows researchers to fully understand the implications of their work and their role within the research, as well as reflect on their own positionality and bias as it pertains to the research they are conducting.

Data Sampling 

The better the sample represents the intended study population, the more likely the researcher is to encompass the varying factors at play. The following are examples of participant sampling and selection: [7]

• Purposive sampling- selection based on the researcher’s rationale in terms of being the most informative.
• Criterion sampling-selection based on pre-identified factors.
• Convenience sampling- selection based on availability.
• Snowball sampling- the selection is by referral from other participants or people who know potential participants.
• Extreme case sampling- targeted selection of rare cases.
• Typical case sampling-selection based on regular or average participants. 

Data Collection and Analysis

Qualitative research uses several techniques including interviews, focus groups, and observation. [1] [2] [3] Interviews may be unstructured, with open-ended questions on a topic and the interviewer adapts to the responses. Structured interviews have a predetermined number of questions that every participant is asked. It is usually one on one and is appropriate for sensitive topics or topics needing an in-depth exploration. Focus groups are often held with 8-12 target participants and are used when group dynamics and collective views on a topic are desired. Researchers can be a participant-observer to share the experiences of the subject or a non-participant or detached observer.

While quantitative research design prescribes a controlled environment for data collection, qualitative data collection may be in a central location or in the environment of the participants, depending on the study goals and design. Qualitative research could amount to a large amount of data. Data is transcribed which may then be coded manually or with the use of Computer Assisted Qualitative Data Analysis Software or CAQDAS such as ATLAS.ti or NVivo. [8] [9] [10]

After the coding process, qualitative research results could be in various formats. It could be a synthesis and interpretation presented with excerpts from the data. [11] Results also could be in the form of themes and theory or model development.

Dissemination

To standardize and facilitate the dissemination of qualitative research outcomes, the healthcare team can use two reporting standards. The Consolidated Criteria for Reporting Qualitative Research or COREQ is a 32-item checklist for interviews and focus groups. [12] The Standards for Reporting Qualitative Research (SRQR) is a checklist covering a wider range of qualitative research. [13]

Examples of Application

Many times a research question will start with qualitative research. The qualitative research will help generate the research hypothesis which can be tested with quantitative methods. After the data is collected and analyzed with quantitative methods, a set of qualitative methods can be used to dive deeper into the data for a better understanding of what the numbers truly mean and their implications. The qualitative methods can then help clarify the quantitative data and also help refine the hypothesis for future research. Furthermore, with qualitative research researchers can explore subjects that are poorly studied with quantitative methods. These include opinions, individual's actions, and social science research.

A good qualitative study design starts with a goal or objective. This should be clearly defined or stated. The target population needs to be specified. A method for obtaining information from the study population must be carefully detailed to ensure there are no omissions of part of the target population. A proper collection method should be selected which will help obtain the desired information without overly limiting the collected data because many times, the information sought is not well compartmentalized or obtained. Finally, the design should ensure adequate methods for analyzing the data. An example may help better clarify some of the various aspects of qualitative research.

A researcher wants to decrease the number of teenagers who smoke in their community. The researcher could begin by asking current teen smokers why they started smoking through structured or unstructured interviews (qualitative research). The researcher can also get together a group of current teenage smokers and conduct a focus group to help brainstorm factors that may have prevented them from starting to smoke (qualitative research).

In this example, the researcher has used qualitative research methods (interviews and focus groups) to generate a list of ideas of both why teens start to smoke as well as factors that may have prevented them from starting to smoke. Next, the researcher compiles this data. The research found that, hypothetically, peer pressure, health issues, cost, being considered “cool,” and rebellious behavior all might increase or decrease the likelihood of teens starting to smoke.

The researcher creates a survey asking teen participants to rank how important each of the above factors is in either starting smoking (for current smokers) or not smoking (for current non-smokers). This survey provides specific numbers (ranked importance of each factor) and is thus a quantitative research tool.

The researcher can use the results of the survey to focus efforts on the one or two highest-ranked factors. Let us say the researcher found that health was the major factor that keeps teens from starting to smoke, and peer pressure was the major factor that contributed to teens to start smoking. The researcher can go back to qualitative research methods to dive deeper into each of these for more information. The researcher wants to focus on how to keep teens from starting to smoke, so they focus on the peer pressure aspect.

The researcher can conduct interviews and/or focus groups (qualitative research) about what types and forms of peer pressure are commonly encountered, where the peer pressure comes from, and where smoking first starts. The researcher hypothetically finds that peer pressure often occurs after school at the local teen hangouts, mostly the local park. The researcher also hypothetically finds that peer pressure comes from older, current smokers who provide the cigarettes.

The researcher could further explore this observation made at the local teen hangouts (qualitative research) and take notes regarding who is smoking, who is not, and what observable factors are at play for peer pressure of smoking. The researcher finds a local park where many local teenagers hang out and see that a shady, overgrown area of the park is where the smokers tend to hang out. The researcher notes the smoking teenagers buy their cigarettes from a local convenience store adjacent to the park where the clerk does not check identification before selling cigarettes. These observations fall under qualitative research.

If the researcher returns to the park and counts how many individuals smoke in each region of the park, this numerical data would be quantitative research. Based on the researcher's efforts thus far, they conclude that local teen smoking and teenagers who start to smoke may decrease if there are fewer overgrown areas of the park and the local convenience store does not sell cigarettes to underage individuals.

The researcher could try to have the parks department reassess the shady areas to make them less conducive to the smokers or identify how to limit the sales of cigarettes to underage individuals by the convenience store. The researcher would then cycle back to qualitative methods of asking at-risk population their perceptions of the changes, what factors are still at play, as well as quantitative research that includes teen smoking rates in the community, the incidence of new teen smokers, among others. [14] [15]

Qualitative research functions as a standalone research design or in combination with quantitative research to enhance our understanding of the world. Qualitative research uses techniques including structured and unstructured interviews, focus groups, and participant observation to not only help generate hypotheses which can be more rigorously tested with quantitative research but also to help researchers delve deeper into the quantitative research numbers, understand what they mean, and understand what the implications are.  Qualitative research provides researchers with a way to understand what is going on, especially when things are not easily categorized. [16]

• Issues of Concern

As discussed in the sections above, quantitative and qualitative work differ in many different ways, including the criteria for evaluating them. There are four well-established criteria for evaluating quantitative data: internal validity, external validity, reliability, and objectivity. The correlating concepts in qualitative research are credibility, transferability, dependability, and confirmability. [4] [11] The corresponding quantitative and qualitative concepts can be seen below, with the quantitative concept is on the left, and the qualitative concept is on the right:

• Internal validity--- Credibility
• External validity---Transferability
• Reliability---Dependability
• Objectivity---Confirmability

In conducting qualitative research, ensuring these concepts are satisfied and well thought out can mitigate potential issues from arising. For example, just as a researcher will ensure that their quantitative study is internally valid so should qualitative researchers ensure that their work has credibility.  

Indicators such as triangulation and peer examination can help evaluate the credibility of qualitative work.

• Triangulation: Triangulation involves using multiple methods of data collection to increase the likelihood of getting a reliable and accurate result. In our above magic example, the result would be more reliable by also interviewing the magician, back-stage hand, and the person who "vanished." In qualitative research, triangulation can include using telephone surveys, in-person surveys, focus groups, and interviews as well as surveying an adequate cross-section of the target demographic.
• Peer examination: Results can be reviewed by a peer to ensure the data is consistent with the findings.

‘Thick’ or ‘rich’ description can be used to evaluate the transferability of qualitative research whereas using an indicator such as an audit trail might help with evaluating the dependability and confirmability.

• Thick or rich description is a detailed and thorough description of details, the setting, and quotes from participants in the research. [5] Thick descriptions will include a detailed explanation of how the study was carried out. Thick descriptions are detailed enough to allow readers to draw conclusions and interpret the data themselves, which can help with transferability and replicability.
• Audit trail: An audit trail provides a documented set of steps of how the participants were selected and the data was collected. The original records of information should also be kept (e.g., surveys, notes, recordings).

One issue of concern that qualitative researchers should take into consideration is observation bias. Here are a few examples:

• Hawthorne effect: The Hawthorne effect is the change in participant behavior when they know they are being observed. If a researcher was wanting to identify factors that contribute to employee theft and tells the employees they are going to watch them to see what factors affect employee theft, one would suspect employee behavior would change when they know they are being watched.
• Observer-expectancy effect: Some participants change their behavior or responses to satisfy the researcher's desired effect. This happens in an unconscious manner for the participant so it is important to eliminate or limit transmitting the researcher's views.
• Artificial scenario effect: Some qualitative research occurs in artificial scenarios and/or with preset goals. In such situations, the information may not be accurate because of the artificial nature of the scenario. The preset goals may limit the qualitative information obtained.
• Clinical Significance

Qualitative research by itself or combined with quantitative research helps healthcare providers understand patients and the impact and challenges of the care they deliver. Qualitative research provides an opportunity to generate and refine hypotheses and delve deeper into the data generated by quantitative research. Qualitative research does not exist as an island apart from quantitative research, but as an integral part of research methods to be used for the understanding of the world around us. [17]

• Enhancing Healthcare Team Outcomes

Qualitative research is important for all members of the health care team as all are affected by qualitative research. Qualitative research may help develop a theory or a model for health research that can be further explored by quantitative research.  Much of the qualitative research data acquisition is completed by numerous team members including social works, scientists, nurses, etc.  Within each area of the medical field, there is copious ongoing qualitative research including physician-patient interactions, nursing-patient interactions, patient-environment interactions, health care team function, patient information delivery, etc. 

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Disclosure: Steven Tenny declares no relevant financial relationships with ineligible companies.

Disclosure: Janelle Brannan declares no relevant financial relationships with ineligible companies.

Disclosure: Grace Brannan declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Tenny S, Brannan JM, Brannan GD. Qualitative Study. [Updated 2022 Sep 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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