review related literature vs review related studies

Similarities and Differences of Review of Related Literature and Review of Related Studies 

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Similarities and Differences of Review of Related Literature and Review of Related Studies

Educational research means the organized collection and examination of the data related to education. It is a scientific study that examines the learning and teaching methods for better understanding of the education system. It is an observation and investigation in the field of education. Research is done in search of new knowledge or to use the existing knowledge in a better way. It helps to acquire useful knowledge and solve the challenges faced in education. Research tries to get a better understanding of education.

Literature review means the overview of the works published previously on a subject matter. It is the summary of the work done by other authors on a topic. Literature review will help a researcher in understanding how to carry on the research and what needs to be covered. 

Similarities between Review of Related Literature and Review of Related Studies

i). Both RRL and RRS is done to understand a subject matter extensively. 

ii). Help an individual to understand their topic of interest in-depth. 

iii). To understand what has already been discovered about a topic and what needs to be researched further. 

Differences of Review of Related Literature and Review of Related Studies

i). Related literature is done from books, professional journals, newspapers, magazines, and other publications. Related studies consist of theses, manuscripts, and dissertations. 

ii). After literature review, the individual tries to develop his/her own opinion on the topic. Review of related studies is obtaining answers from what has been studied. 

iii). Related literature focuses on the opinions and ideas of one’s own on a particular topic. Related studies analyze the work of other researchers and focus on the results received by them.  

• University Libraries
• Research Guides
• Reviewing Research: Literature Reviews, Scoping Reviews, Systematic Reviews
• Differentiating the Three Review Types

Reviewing Research: Literature Reviews, Scoping Reviews, Systematic Reviews: Differentiating the Three Review Types

• Framework, Protocol, and Writing Steps
• Working with Keywords/Subject Headings
• Citing Research

The Differences in the Review Types

Grant, M.J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. H ealth Information & Libraries Journal , 26: 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x   The objective of this study is to provide descriptive insight into the most common types of reviews, with illustrative examples from health and health information domains.

• What Type of Review is Right for you (Cornell University)

Literature Reviews

Literature Review: it is a product and a process.

As a product , it is a carefully written examination, interpretation, evaluation, and synthesis of the published literature related to your topic. It focuses on what is known about your topic and what methodologies, models, theories, and concepts have been applied to it by others.

The process is what is involved in conducting a review of the literature.

• It is ongoing
• It is iterative (repetitive)
• It involves searching for and finding relevant literature.
• It includes keeping track of your references and preparing and formatting them for the bibliography of your thesis

• Literature Reviews (University of North Carolina at Chapel Hill) This handout will explain what literature reviews are and offer insights into the form and construction of literature reviews in the humanities, social sciences, and sciences.

Scoping Reviews

Scoping reviews are a " preliminary assessment of potential size and scope of available research literature . Aims to identify nature and extent of research evidence (usually including ongoing research)." Grant and Booth (2009).

Scoping reviews are not mapping reviews: Scoping reviews are more topic based and mapping reviews are more question based.

• examining emerging evidence when specific questions are unclear - clarify definitions and conceptual boundaries
• identify and map the available evidence
• a scoping review is done prior to a systematic review
• to summarize and disseminate research findings in the research literature
• identify gaps with the intention of resolution by future publications

• Scoping review timeframe and limitations (Touro College of Pharmacy

Systematic Reviews

Many evidence-based disciplines use ‘systematic reviews," this type of review is a specific methodology that aims to comprehensively identify all relevant studies on a specific topic, and to select appropriate studies based on explicit criteria . ( https://cebma.org/faq/what-is-a-systematic-review/ )

• clearly defined search criteria
• an explicit reproducible methodology
• a systematic search of the literature with the defined criteria met
• assesses validity of the findings - no risk of bias
• a comprehensive report on the findings, apparent transparency in the results

• Better evidence for a better world Browsable collection of systematic reviews
• Systematic Reviews in the Health Sciences by Molly Maloney Last Updated Jan 8, 2024 368 views this year
• Next: Framework, Protocol, and Writing Steps >>

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Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
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• Choosing a Title
• Making an Outline
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• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
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• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
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• Insiderness
• Using Non-Textual Elements
• Limitations of the Study
• Common Grammar Mistakes
• Writing Concisely
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• Further Readings
• Generative AI and Writing
• USC Libraries Tutorials and Other Guides
• Bibliography

A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

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 primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the 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 a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded 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 make summary claims of the sort found in systematic reviews [see below].

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 or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus 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 [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon 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. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own 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 analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . 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?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to 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.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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Research Methods

• Getting Started
• Literature Review Research
• Research Design
• Research Design By Discipline
• SAGE Research Methods
• 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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• Last Updated: Aug 21, 2023 4:07 PM
• URL: https://guides.lib.udel.edu/researchmethods

Literature reviews as independent studies: guidelines for academic practice

• Original Paper
• Open access
• Published: 14 October 2022
• Volume 16 , pages 2577–2595, ( 2022 )

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• Sascha Kraus   ORCID: orcid.org/0000-0003-4886-7482 1 , 2 ,
• Matthias Breier 3 ,
• Weng Marc Lim 4 , 8 , 22 ,
• Marina Dabić 5 , 6 ,
• Satish Kumar 7 , 8 ,
• Dominik Kanbach 9 , 10 ,
• Debmalya Mukherjee 11 ,
• Vincenzo Corvello 12 ,
• Juan Piñeiro-Chousa 13 ,
• Eric Liguori 14 ,
• Daniel Palacios-Marqués 15 ,
• Francesco Schiavone 16 , 17 ,
• Alberto Ferraris 18 , 21 ,
• Cristina Fernandes 19 , 20 &
• João J. Ferreira 19  

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Review articles or literature reviews are a critical part of scientific research. While numerous guides on literature reviews exist, these are often limited to the philosophy of review procedures, protocols, and nomenclatures, triggering non-parsimonious reporting and confusion due to overlapping similarities. To address the aforementioned limitations, we adopt a pragmatic approach to demystify and shape the academic practice of conducting literature reviews. We concentrate on the types, focuses, considerations, methods, and contributions of literature reviews as independent, standalone studies. As such, our article serves as an overview that scholars can rely upon to navigate the fundamental elements of literature reviews as standalone and independent studies, without getting entangled in the complexities of review procedures, protocols, and nomenclatures.

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1 Introduction

A literature review – or a review article – is “a study that analyzes and synthesizes an existing body of literature by identifying, challenging, and advancing the building blocks of a theory through an examination of a body (or several bodies) of prior work (Post et al. 2020 , p. 352). Literature reviews as standalone pieces of work may allow researchers to enhance their understanding of prior work in their field, enabling them to more easily identify gaps in the body of literature and potential avenues for future research. More importantly, review articles may challenge established assumptions and norms of a given field or topic, recognize critical problems and factual errors, and stimulate future scientific conversations around that topic. Literature reviews Footnote 1 come in many different formats and purposes:

Some review articles conduct a critical evaluation of the literature, whereas others elect to adopt a more exploratory and descriptive approach.

Some reviews examine data, methodologies, and findings, whereas others look at constructs, themes, and theories.

Some reviews provide summaries by holistically synthesizing the existing research on a topic, whereas others adopt an integrative approach by assessing related and interdisciplinary work.

The number of review articles published as independent or standalone studies has been increasing over time. According to Scopus (i.e., search database ), reviews (i.e., document type ) were first published in journals (i.e., source type ) as independent studies in 1945, and they subsequently appeared in three digits yearly from the late 1980s to the late 1990s, four digits yearly from the early 2000s to the late 2010s, and five digits in the year 2021 (Fig.  1 ). This increase is indicative that reviewers and editors in business and management research alike see value and purpose in review articles to such a level that they are now commonly accepted as independent, standalone studies. This development is also reflected in the fact that some academic journals exclusively publish review articles (e.g., the Academy of Management Annals , or the  International Journal of Management Reviews ), and journals publishing in various fields often have special issues dedicated to literature reviews on certain topic areas (e.g., the Journal of Management and the Journal of International Business Studies ).

Full-year publication trend of review articles on Scopus (1945–2021)

One of the most important prerequisites of a high-quality review article is that the work follows an established methodology, systematically selects and analyzes articles, and periodically covers the field to identify latest developments (Snyder 2019 ). Additionally, it needs to be reproducible, well-evidenced, and transparent, resulting in a sample inclusive of all relevant and appropriate studies (Gusenbauer and Haddaway 2020; Hansen et al. 2021 ). This observation is in line with Palmatier et al. ( 2018 ), who state that review articles provide an important synthesis of findings and perspectives in a given body of knowledge. Snyder ( 2019 ) also reaffirmed this rationale, pointing out that review articles have the power to answer research questions beyond that which can be achieved in a single study. Ultimately, readers of review articles stand to gain a one-stop, state-of-the-art synthesis (Lim et al. 2022a ; Popli et al. 2022) that encapsulates critical insights through the process of re-interpreting, re-organizing, and re-connecting a body knowledge (Fan et al. 2022 ).

There are many reasons to conduct review articles. Kraus et al. ( 2020 ) explicitly mention the benefits of conducting systematic reviews by declaring that they often represent the first step in the context of larger research projects, such as doctoral dissertations. When carrying out work of this kind, it is important that a holistic overview of the current state of literature is achieved and embedded into a proper synthesis. This allows researchers to pinpoint relevant research gaps and adequately fit future conceptual or empirical studies into the state of the academic discussion (Kraus et al., 2021 ). A review article as an independent or standalone study is a viable option for any academic – especially young scholars, such as doctoral candidates – who wishes to delve into a specific topic for which a (recent) review article is not available.

The process of conducting a review article can be challenging, especially for novice scholars (Boell and Cecez-Kecmanovic 2015 ). Therefore, it is not surprising that numerous guides have been written in an attempt to improve the quality of review studies and support emerging scholars in their endeavors to have their work published. These guides for conducting review articles span a variety of academic fields, such as engineering education (Borrego et al. 2014 ), health sciences (Cajal et al. 2020 ), psychology (Laher and Hassem 2020 ), supply chain management (Durach et al. 2017 ), or business and entrepreneurship (Kraus et al. 2020 ; Tranfield et al. 2003 ) – the latter were among the first scholars to recognize the need to educate business/management scholars on the roles of review studies in assembling, ascertaining, and assessing the intellectual territory of a specific knowledge domain. Furthermore, they shed light on the stages (i.e., planning the review, conducting the review, reporting, and dissemination) and phases (i.e., identifying the need for a review, preparation of a proposal for a review, development of a review protocol, identification of research, selection of studies, study quality assessment, data extraction and monitoring progress, data synthesis, the report and recommendations, and getting evidence into practice) of conducting a systematic review. Other scholars have either adapted and/or developed new procedures (Kraus et al. 2020 ; Snyder 2019 ) or established review protocols such as the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram (Moher et al. 2015 ). The latter provides a checklist that improves transparency and reproducibility, thus reducing questionable research practices. The declarative and procedural knowledge of a checklist allows users to derive value from (and, in some cases, produce) methodological literature reviews.

Two distinct and critical gaps or issues provide impetus for our article. First, while the endeavors of the named scholars are undoubtedly valuable contributions, they often encourage other scholars to explain the methodology of their review studies in a non-parsimonious way ( 1st issue ). This can become problematic if this information distracts and deprives scholars from providing richer review findings, particularly in instances in which publication outlets impose a strict page and/or word limit. More often than not, the early parts (i.e., stages/phases, such as needs, aims, and scope) of these procedures or protocols are explained in the introduction, but they tend to be reiterated in the methodology section due to the prescription of these procedures or protocols. Other parts of these procedures or protocols could also be reported more parsimoniously, for example, by filtering out documents, given that scientific databases (such as Scopus or Web of Science ) have since been upgraded to allow scholars to select and implement filtering criteria when conducting a search (i.e., criterion-by-criterion filtering may no longer be necessary). More often than not, the procedures or protocols of review studies can be signposted (e.g., bracket labeling) and disclosed in a sharp and succinct manner while maintaining transparency and replicability.

Other guides have been written to introduce review nomenclatures (i.e., names/naming) and their equivalent philosophical underpinnings. Palmatier et al. ( 2018 ) introduced three clearly but broadly defined nomenclatures of literature reviews as independent studies: domain-based reviews, theory-based reviews, and method-based reviews. However, such review nomenclatures can be confusing due to their overlapping similarities ( 2nd issue ). For example, Lim et al. ( 2022a ) highlighted their observation that the review nomenclatures associated with domain-based reviews could also be used for theory-based and method-based reviews.

The two aforementioned issues – i.e., the lack of a parsimonious understanding and the reporting of the review methodology , and the confusion emerging from review nomenclatures – are inarguably the unintended outcomes of diving into an advanced (i.e., higher level) understanding of literature review procedures, protocols, and nomenclatures from a philosophical perspective (i.e., underpinnings) without a foundational (i.e., basic level) understanding of the fundamental (i.e., core) elements of literature reviews from a pragmatic perspective. Our article aims to shed light on these issues and hopes to provide clarity for future scholarly endeavors.

Having a foundational understanding of literature reviews as independent studies is (i) necessary when addressing the aforementioned issues; (ii) important in reconciling and scaffolding our understanding, and (iii) relevant and timely due to the proliferation of literature reviews as independent studies. To contribute a solution toward addressing this gap , we aim to demystify review articles as independent studies from a pragmatic standpoint (i.e., practicality). To do so, we deliberately (i) move away from review procedures, protocols, and nomenclatures, and (ii) invest our attention in developing a parsimonious, scaffolded understanding of the fundamental elements (i.e., types, focuses, considerations, methods, and contributions) of review articles as independent studies.

Three contributions distinguish our article. It is worth noting that pragmatic guides (i.e., foundational knowledge), such as the present one, are not at odds with extant philosophical guides (i.e., advanced knowledge), but rather they complement them. Having a foundational knowledge of the fundamental elements of literature reviews as independent studies is valuable , as it can help scholars to (i) gain a good grasp of the fundamental elements of literature reviews as independent studies ( 1st contribution ), and (ii) mindfully adopt or adapt existing review procedures, protocols, and nomenclatures to better suit the circumstances of their reviews (e.g., choosing and developing a well-defined review nomenclature, and choosing and reporting on review considerations and steps more parsimoniously) ( 2nd contribution ). Therefore, this pragmatic guide serves as (iii) a foundational article (i.e., preparatory understanding) for literature reviews as independent studies ( 3rd contribution ). Following this, extant guides using a philosophical approach (i.e., advanced understanding) could be relied upon to make informed review decisions (e.g., adoption, adaptation) in response to the conventions of extant review procedures, protocols, and nomenclatures (Fig.  2 ).

Foundational and advanced understanding of literature reviews as independent studies

2 Fundamental elements of literature reviews as independent studies

A foundational understanding of literature reviews as independent studies can be acquired through the appreciation of five fundamental elements – i.e., types, focuses, considerations, methods, and contributions – which are illustrated in Fig.  3 and summarized in the following sections.

Fundamental elements of literature reviews as independent studies

There are two types of literature reviews as independent studies: systematic literature reviews ( SLRs ) and non-systematic literature reviews ( non-SLRs ). It is important to recognize that SLRs and non-SLRs are not review nomenclatures (i.e., names/naming) but rather review types (i.e., classifications).

In particular, SLRs are reviews carried out in a systematic way using an adopted or adapted procedure or protocol to guide data curation and analysis, thus enabling transparent disclosure and replicability (Lim et al. 2022a ; Kraus et al. 2020 ). Therefore, any review nomenclature guided by a systematic methodology is essentially an SLR. The origin of this type of literature review can be traced back to the evidence-based medicine movement in the early 1990s, with the objective being to overcome the issue of inconclusive findings in studies for medical treatments (Boell and Cecez-Kecmanovic 2015 ).

In contrast, non-SLRs are reviews conducted without any systematic procedure or protocol; instead, they weave together relevant literature based on the critical evaluations and (subjective) choices of the author(s) through a process of discovery and critique (e.g., pointing out contradictions and questioning assertions or beliefs); they are shaped by the exposure, expertise, and experience (i.e., the “3Es” in judgement calls) of the author(s). Therefore, non-SLRs are essentially critical reviews of the literature (Lim and Weissmann 2021 ).

2.2 Focuses

Unlike Palmatier et al. ( 2018 ) who considered domain-based reviews, theory-based reviews, and method-based reviews as review nomenclatures, we consider domain , theory , and method as three substantive focuses that can take center stage in literature reviews as independent studies. This is in line with our attempt to move away from review nomenclatures when providing a foundational understanding of literature reviews as independent studies.

A review that is domain-focused can examine: (i) a  concept (e.g., customer engagement; Lim et al. 2022b ; digital transformation; Kraus et al. 2021 ; home sharing; Lim et al. 2021 ; sharing economy; Lim 2020 ), (ii) a context (e.g., India; Mukherjee et al. 2022a ), (iii) a discipline (e.g., entrepreneurship; Ferreira et al. 2015 ; international business; Ghauri et al. 2021 ), (iv) a field (e.g., family business; Lahiri et al. 2020 ; Rovelli et al. 2021 ; female entrepreneurship; Ojong et al. 2021 ), or (v) an outlet (e.g., Journal of Business Research ; Donthu et al. 2020 ; Management International Review ; Mukherjee et al. 2021 ; Review of Managerial Science ; Mas-Tur et al. 2020 ), which typically offer broad, overarching insights.

Domain-focused hybrids , such as the between-domain hybrid (e.g., concept-discipline hybrid, such as digital transformation in business and management; Kraus et al. 2022 ; religion in business and entrepreneurship; Kumar et al. 2022a ; personality traits in entrepreneurship; Salmony and Kanbach 2022 ; and policy implications in HR and OB research; Aguinis et al., 2022 ) and the within-domain hybrid (e.g., the concept-concept hybrid, such as customer engagement and social media; Lim and Rasul 2022 ; and global business and organizational excellence; Lim 2022 ; and the discipline-discipline hybrid, such as neuromarketing; Lim 2018 ) are also common as they can provide finer-grained insights.

A review that is theory-focused can explore a standalone theory (e.g., theory of planned behavior; Duan and Jiang 2008 ), as well as a theory in conjunction with a domain , such as the concept-theory hybrid (e.g., behavioral control and theory of planned behavior; Lim and Weissmann 2021 ) and the theory-discipline hybrid (e.g., theory of planned behavior in hospitality, leisure, and tourism; Ulker-Demirel and Ciftci 2020 ), or a theory in conjunction with a method (e.g., theory of planned behavior and structural equation modeling).

A review that is method-focused can investigate a standalone method (e.g., structural equation modeling; Deng et al. 2018 ) or a method in conjunction with a domain , such as the method-discipline hybrid (e.g., fsQCA in business and management; Kumar et al. 2022b ).

2.3 Planning the review, critical considerations, and data collection

The considerations required for literature reviews as independent studies depend on their type: SLRs or non-SLRs.

For non-SLRs, scholars often rely on the 3Es (i.e., exposure, expertise, and experience) to provide a critical review of the literature. Scholars who embark on non-SLRs should be well versed with the literature they are dealing with. They should know the state of the literature (e.g., debatable, underexplored, and well-established knowledge areas) and how it needs to be deciphered (e.g., tenets and issues) and approached (e.g., reconciliation proposals and new pathways) to advance theory and practice. In this regard, non-SLRs follow a deductive reasoning approach, whereby scholars initially develop a set of coverage areas for reviewing a domain, theory, or method and subsequently draw on relevant literature to shed light and support scholarly contentions in each area.

For SLRs, scholars often rely on a set of criteria to provide a well-scoped (i.e., breadth and depth), structured (i.e., organized aspects), integrated (i.e., synthesized evidence) and interpreted/narrated (i.e., describing what has happened, how and why) systematic review of the literature. Footnote 2 In this regard, SLRs follow an inductive reasoning approach, whereby a set of criteria is established and implemented to develop a corpus of scholarly documents that scholars can review. They can then deliver a state-of-the-art overview, as well as a future agenda for a domain, theory, or method. Such criteria are often listed in philosophical guides on SLR procedures (e.g., Kraus et al. 2020 ; Snyder 2019 ) and protocols (e.g., PRISMA), and they may be adopted/adapted with justifications Footnote 3 . Based on their commonalities they can be summarized as follows:

Search database (e.g., “Scopus” and/or “Web of Science”) can be defined based on justified evidence (e.g., by the two being the largest scientific databases of scholarly articles that can provide on-demand bibliographic data or records; Pranckutė 2021 ). To avoid biased outcomes due to the scope covered by the selected database, researchers could utilize two or more different databases (Dabić et al. 2021 ).

Search keywords may be developed by reading scholarly documents and subsequently brainstorming with experts. The expanding number of databases, journals, periodicals, automated approaches, and semi-automated procedures that use text mining and machine learning can offer researchers the ability to source new, relevant research and forecast the citations of influential studies. This enables them to determine further relevant articles.

Boolean operators (e.g., AND, OR) should be strategically used in developing the  string   of search keywords (e.g., “engagement” AND “customer” OR “consumer” OR “business”). Furthermore, the correct and precise application of quotation marks is important but is very frequently sidestepped, resulting in incorrect selection processes and differentiated results.

Search period (e.g., between a specified period [e.g., 2000 to 2020] or up to the latest full year at the time or writing [e.g., up to 2021]) can be defined based on the justified scope of study (e.g., contemporary evolution versus historical trajectory).

Search field (e.g., “article title, abstract, keywords”) can be defined based on justified assumptions (e.g., it is assumed that the focus of relevant documents will be mentioned in the article title, abstract, and/or keywords).

Subject area (e.g., “business, management, and accounting”) can be defined based on justified principles (e.g., the focus of the review is on the marketing discipline, which is located under the “business, management, and accounting” subject area in Scopus).

Publication stage (e.g., “final”) can be defined based on justified grounds (e.g., enabling greater accuracy in replication).

Document type (e.g., “article” and/or “review”), which reflects the type of scientific/practical contributions (e.g., empirical, synthesis, thought), can be defined based on justified rationales (e.g., articles selected because they are peer-reviewed; editorials not selected because they are not peer-reviewed).

Source type (e.g., “journal”) can be defined based on justified reasons (e.g., journals selected because they publish finalized work; conference proceedings not selected because they are work in progress, and in business/management, they are usually not being considered as full-fledged “publications”).

Language (e.g., “English”) can be determined based on justified limitations (e.g., nowadays, there are not many reasons to use another language besides the academic lingua franca English). Different spellings should also be considered, as the literature may contain both American and British spelling variants (e.g., organization and organisation). Truncation and wildcards in searches are recommended to capture both sets of spellings. It is important to note that each database varies in its symbology.

Quality filtering (e.g., “A*” and “A” or “4*”, “4”, and “3”) can be defined based on justified motivations (e.g., the goal is to unpack the most originally and rigorously produced knowledge, which is the hallmark of premier journals, such as those ranked “A*” and “A” by the Australian Business Deans Council [ABDC] Journal Quality List [JQL] and rated “4*”, “4”, and “3” by the Chartered Association of Business Schools [CABS] Academic Journal Guide [AJG]).

Document relevance (i.e., within the focus of the review) can be defined based on justified judgement (e.g., for a review focusing on customer engagement, articles that mention customer engagement as a passing remark without actually investigating it would be excluded).

Others: Screening process should be accomplished by beginning with the deduction of duplicate results from other databases, tracked using abstract screening to exclude unfitting studies, and ending with the full-text screening of the remaining documents.

Others: Exclusion-inclusion criteria interpretation of the abstracts/articles is obligatory when deciding whether or not the articles dealt with the matter. This step could involve removing a huge percentage of initially recognized articles.

Others: Codebook building pertains to the development of a codebook of the main descriptors within a specific field. An inductive approach can be followed and, in this case, descriptors are not established beforehand. Instead, they are established through the analysis of the articles’ content. This procedure is made up of several stages: (i) the extraction of important content from titles, abstracts, and keywords; (ii) the classification of this content to form a reduced list of the core descriptors; and (iii) revising the codebook in iterations and combining similar categories, thus developing a short list of descriptors (López-Duarte et al. 2016 , p. 512; Dabić et al. 2015 ; Vlacic et al. 2021 ).

2.4 Methods

Various methods are used to analyze the pertinent literature. Often, scholars choose a method for corpus analysis before corpus curation. Knowing the analytical technique beforehand is useful, as it allows researchers to acquire and prepare the right data in the right format. This typically occurs when scholars have decided upon and justified pursuing a specific review nomenclature upfront (e.g., bibliometric reviews) based on the problem at hand (e.g., broad domain [outlet] with a large corpus [thousands of articles], such as a premier journal that has been publishing for decades) (Donthu et al. 2021 ). However, this may not be applicable in instances where (i) scholars do not curate a corpus of articles (non-SLRs), and (ii) scholars only know the size of the corpus of articles once that corpus is curated (SLRs). Therefore, scholars may wish to decide on a method of analyzing the literature depending on (i) whether they rely on a corpus of articles (i.e., yes or no), and (ii) the size of the corpus of articles that they rely on to review the literature (i.e., n  = 0 to ∞).

When analytical techniques (e.g., bibliometric analysis, critical analysis, meta-analysis) are decoupled from review nomenclatures (e.g., bibliometric reviews, critical reviews, meta-analytical reviews), we uncover a toolbox of the following methods for use when analyzing the literature:

Bibliometric analysis measures the literature and processes data by using algorithm, arithmetic, and statistics to analyze, explore, organize, and investigate large amounts of data. This enables scholars to identify and recognize potential “hidden patterns” that could help them during the literature review process. Bibliometrics allows scholars to objectively analyze a large corpus of articles (e.g., high hundreds or more) using quantitative techniques (Donthu et al. 2021 ). There are two overarching categories for bibliometric analysis: performance analysis and science mapping. Performance analysis enables scholars to assess the productivity (publication) and impact (citation) of the literature relating to a domain, method, or theory using various quantitative metrics (e.g., average citations per publication or year, h -index, g -index, i -index). Science mapping grants scholars the ability to map the literature in that domain, method, or theory based on bibliographic data (e.g., bibliographic coupling generates thematic clusters based on similarities in shared bibliographic data [e.g., references] among citing articles; co-citation analysis generates thematic clusters based on commonly cited articles; co-occurrence analysis generates thematic clusters based on bibliographic data [e.g., keywords] that commonly appear together; PageRank analysis generates thematic clusters based on articles that are commonly cited in highly cited articles; and topic modeling generates thematic clusters based on the natural language processing of bibliographic data [e.g., article title, abstract, and keywords]). Footnote 4 Given the advancement in algorithms and technology, reviews using bibliometric analysis are considered to be smart (Kraus et al. 2021 ) and technologically-empowered (Kumar et al. 2022b ) SLRs, in which a review has harnessed the benefits of (i) the machine learning of the bibliographic data of scholarly research from technologically-empowered scientific databases, and (ii) big data analytics involving various science mapping techniques (Kumar et al. 2022c ).

Content analysis allows scholars to analyze a small to medium corpus of articles (i.e., tens to low hundreds) using quantitative and qualitative techniques. From a quantitative perspective , scholars can objectively carry out a content analysis by quantifying a specific unit of analysis . A useful method of doing so involves adopting, adapting, or developing an organizing framework . For example, Lim et al. ( 2021 ) employed an organizing (ADO-TCM) framework to quantify content in academic literature based on: (i) the categories of knowledge; (ii) the relationships between antecedents, decisions, and outcomes; and (iii) the theories, contexts, and methods used to develop the understanding for (i) and (ii). The rapid evolution of software for content analysis allows scholars to carry out complex elaborations on the corpus of analyzed articles, so much so that the most recent software enables the semi-automatic development of an organizing framework (Ammirato et al. 2022 ). From a qualitative perspective , scholars can conduct a content analysis or, more specifically, a thematic analysis , by subjectively organizing the content into themes. For example, Creevey et al. ( 2022 ) reviewed the literature on social media and luxury, providing insights on five core themes (i.e., luxury brand strategy, luxury brand social media communications, luxury consumer attitudes and perceptions, engagement, and the influence of social media on brand performance-related outcomes) generated through a content (thematic) analysis. Systematic approaches for inductive concept development through qualitative research are similarly applied in literature reviews in an attempt to reduce the subjectivity of derived themes. Following the principles of the approach by Gioia et al. ( 2012 ), Korherr and Kanbach ( 2021 ) develop a taxonomy of human-related capabilities in big data analytics. Building on a sample of 75 studies for the literature review, 33 first-order concepts are identified. These are categorized into 15 second-order themes and are finally merged into five aggregate dimensions. Using the same procedure, Leemann and Kanbach ( 2022 ) identify 240 idiosyncratic dynamic capabilities in a sample of 34 studies for their literature review. They then categorize these into 19 dynamic sub-capabilities. The advancement of technology also makes it possible to conduct content analysis using computer assisted qualitative data analysis (CAQDA) software (e.g., ATLAS.ti, Nvivo, Quirkos) (Lim et al. 2022a ).

Critical analysis allows scholars to subjectively use their 3Es (i.e., exposure, expertise, and experience) to provide a critical evaluation of academic literature. This analysis is typically used in non-SLRs, and can be deployed in tandem with other analyses, such as bibliometric analysis and content analysis in SLRs, which are used to discuss consensual, contradictory, and underexplored areas of the literature. For SLRs, scholars are encouraged to engage in critical evaluations of the literature so that they can truly contribute to advancing theory and practice (Baker et al. 2022 ; Lim et al. 2022a ; Mukherjee et al. 2022b ).

Meta-analysis allows scholars to objectively establish a quantitative estimate of commonly studied relationships in the literature (Grewal et al. 2018 ). This analysis is typically employed in SLRs intending to reconcile a myriad of relationships (Lim et al. 2022a ). The relationships established are often made up of conflicting evidence (e.g., a positive or significant effect in one study, but a negative or non-significant effect in another study). However, through meta-analysis, scholars are able to identify potential factors (e.g., contexts or sociodemographic information) that may have led to the conflict.

Others: Multiple correspondence analysis helps to map the field, assessing the associations between qualitative content within a matrix of variables and cases. Homogeneity Analysis by Means of Alternating Least Squares ( HOMALS ) is also considered useful in allowing researchers to map out the intellectual structure of a variety of research fields (Gonzalez-Loureiro et al. 2015 ; Gonzalez-Louriero 2021; Obradović et al. 2021 ). HOMALS can be performed in R or used along with a matrix through SPSS software. In summary, the overall objective of this analysis is to discover a low dimensional representation of the original high dimensional space (i.e., the matrix of descriptors and articles). To measure the goodness of fit, a loss function is used. This function is used minimally, and the HOMALS algorithm is applied to the least squares loss functions in SPSS. This analysis provides a proximity map, in which articles and descriptors are shown in low-dimensional spaces (typically on two axes). Keywords are paired and each couple that appears together in a large number of articles is shown to be closer on the map and vice-versa.

When conducting a literature review, software solutions allow researchers to cover a broad range of variables, from built-in functions of statistical software packages to software orientated towards meta-analyses, and from commercial to open-source solutions. Personal preference plays a huge role, but the decision as to which software will be the most useful is entirely dependent on how complex the methods and the dataset are. Of all the commercial software providers, we have found the built-in functions of (i) R and VOSviewer most useful in performing bibliometric analysis (Aria and Cuccurullo 2017 ; R Core Team 2021 ; Van Eck and Waltman 2014 ) and (ii) Stata most useful in performing meta-analytical tasks.

Many different analytical tools have been used. These include simple document counting, citation analysis, word frequency analysis, cluster analysis, co-word analysis, and cooperation analysis (Daim et al. 2006 ). Software has also been produced for bibliometric analysis, such as the Thomson Data Analyzer (TDA), which Thomson Reuters created, and CiteSpace developed by Chen ( 2013 ). VOSviewer helps us to construct and visualize bibliometric networks, which can include articles, journals, authors, countries, and institutions, among others (Van Eck and Waltman 2014 ). These can be organized based on citations, co-citations, bibliographic coupling, or co-authorship relations. In addition, VOSviewer provides text mining functions, which can be used to facilitate a better understanding of co-occurrence networks with regards to the key terms taken from a body of scientific literature (Donthu et al. 2021 ; Wong 2018 ). Other frequently used tools include for bibliometric analysis include Bibliometrix/Biblioshiny in R, CitNetExplorer, and Gephi, among others.

2.5 Contributions

Well-conducted literature reviews may make multiple contributions to the literature as standalone, independent studies.

Generally, there are three primary contributions of literature reviews as independent studies: (i) to provide an overview of current knowledge in the domain, method, or theory, (ii) to provide an evaluation of knowledge progression in the domain, method, or theory, including the establishment of key knowledge, conflicting or inconclusive findings, and emerging and underexplored areas, and (iii) to provide a proposal for potential pathways for advancing knowledge in the domain, method, or theory (Lim et al. 2022a , p. 487). Developing theory through literature reviews can take many forms, including organizing and categorizing the literature, problematizing the literature, identifying and exposing contradictions, developing analogies and metaphors, and setting out new narratives and conceptualizations (Breslin and Gatrell 2020 ). Taken collectively, these contributions offer crystalized, evidence-based insights that both ‘mine’ and ‘prospect’ the literature, highlighting extant gaps and how they can be resolved (e.g., flags paradoxes or theoretical tensions, explaining why something has not been done, what the challenges are, and how these challenges can be overcome). These contributions can be derived through successful bibliometric analysis, content analysis, critical analysis, and meta-analysis.

Additionally, the deployment of specific methods can bring in further added value. For example, a performance analysis in a bibliometric analysis can contribute to: (i) objectively assessing and reporting research productivity and impact ; (ii) ascertaining reach for coverage claims ; (iii) identifying social dominance and hidden biases ; (iv) detecting anomalies ; and (v) evaluating ( equitable ) relative performance ; whereas science mapping in bibliometric analysis can contribute to: (i) objectively discovering thematic clusters of knowledge ; (ii) clarifying nomological networks ; (iii) mapping social patterns ; (iv) tracking evolutionary nuances ; and (v) recognizing knowledge gaps (Mukherjee et al. 2022b , p. 105).

3 Conclusion

Independent literature reviews will continue to be written as a result of their necessity, importance, relevance, and urgency when it comes to advancing knowledge (Lim et al. 2022a ; Mukherjee et al. 2022b ), and this can be seen in the increasing number of reviews being published over the last several years. Literature reviews advance academic discussion. Journal publications on various topics and subject areas are becoming more frequent sites for publication. This trend will only heighten the need for literature reviews. This article offers directions and control points that address the needs of three different stakeholder groups: producers (i.e., potential authors), evaluators (i.e., journal editors and reviewers), and users (i.e., new researchers looking to learn more about a particular methodological issue, and those teaching the next generation of scholars). Future producers will derive value from this article’s teachings on the different fundamental elements and methodological nuances of literature reviews. Procedural knowledge (i.e., using control points to assist in decision-making during the manuscript preparation phase) will also be of use. Evaluators will be able to make use of the procedural and declarative knowledge evident in control points as well. As previously outlined, the need to cultivate novelty within research on business and management practices is vital. Scholars must also be supported to choose not only safe mining approaches; they should also be encouraged to attempt more challenging and risky ventures. It is important to note that abstracts often seem to offer a lot of potential, stating that authors intend to make large conceptual contributions, broadening the horizons of the field.

Our article offers important insights also for practitioners. Noteworthily, our framework can support corporate managers in decomposing and better understanding literature reviews as ad-hoc and independent studies about specific topics that matter for their organization. For instance, practitioners can understand more easily what are the emerging trends within their domain of interest and make corporate decisions in line with such trends.

This article arises from an intentional decoupling from philosophy, in favor of adopting a more pragmatic approach. This approach can assist us in clarifying the fundamental elements of literature reviews as independent studies. Five fundamental elements must be considered: types, focuses, considerations, methods, and contributions. These elements offer a useful frame for scholars starting to work on a literature review. Overview articles (guides) such as ours are thus invaluable, as they equip scholars with a solid foundational understanding of the integral elements of a literature review. Scholars can then put these teachings into practice, armed with a better understanding of the philosophy that underpins the procedures, protocols, and nomenclatures of literature reviews as independent studies.

Data availability

Our manuscript has no associate data.

Our focus here is on standalone literature reviews in contrast with literature reviews that form the theoretical foundation for a research article.

Scoping reviews, structured reviews, integrative reviews, and interpretive/narrative reviews are commonly found in review nomenclature. However, the philosophy of these review nomenclatures essentially reflects what constitutes a good SLR. That is to say, a good SLR should be well scoped, structured, integrated, and interpreted/narrated. This observation reaffirms our position and the value of moving away from review nomenclatures to gain a foundational understanding of literature reviews as independent studies.

Given that many of these considerations can be implemented simultaneously in contemporary versions of scientific databases, scholars may choose to consolidate them into a single (or a few) step(s), where appropriate, so that they can be reported more parsimoniously. For a parsimonious but transparent and replicable exemplar, see Lim ( 2022 ).

Where keywords are present (e.g., author keywords or keywords derived from machine learning [e.g., natural language processing]), it is assumed that each keyword represents a specific meaning (e.g., topic [concept, context], method), and that a collection of keywords grouped under the same cluster represents a specific theme.

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Kraus, S., Breier, M., Lim, W.M. et al. Literature reviews as independent studies: guidelines for academic practice. Rev Manag Sci 16 , 2577–2595 (2022). https://doi.org/10.1007/s11846-022-00588-8

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A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

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3 Conducting a Review of Related Literature

• Published: February 2023
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This chapter explores approaches for reviewing and synthesizing research literature. Authors conduct reviews of literature in order to provide a context within which to place their study. A literature review can provide background information necessary for a reader to understand a study. Furthermore, syntheses of findings from related literature will reveal what is known about a topic and allow researchers to identify the gaps or weaknesses in the knowledge base, which can help to establish a rationale for a study. Quantitative, qualitative, and action research studies always include a review of related literature; historical and philosophical studies may not. Modern search engines make finding related literature easy but correspondingly difficult in that they return so many possibilities that it can take considerable time and effort to sort through and identify the most relevant sources. Using a reference management system can save many hours of labor in organizing and formatting references. Organizing information under various headings based on important elements of a study will help the reader understand how previous studies are related to the current study. A literature review should also entail critique; researchers are expected to point out weaknesses in data collection or analysis or discrepancies among various findings. Most important, a researcher must demonstrate how previous research is connected to their own. Perhaps they are attempting to fill in gaps in the literature, extend previous findings, or contradict prevailing notions.

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• Korean J Anesthesiol
• v.71(2); 2018 Apr

Introduction to systematic review and meta-analysis

1 Department of Anesthesiology and Pain Medicine, Inje University Seoul Paik Hospital, Seoul, Korea

2 Department of Anesthesiology and Pain Medicine, Chung-Ang University College of Medicine, Seoul, Korea

Systematic reviews and meta-analyses present results by combining and analyzing data from different studies conducted on similar research topics. In recent years, systematic reviews and meta-analyses have been actively performed in various fields including anesthesiology. These research methods are powerful tools that can overcome the difficulties in performing large-scale randomized controlled trials. However, the inclusion of studies with any biases or improperly assessed quality of evidence in systematic reviews and meta-analyses could yield misleading results. Therefore, various guidelines have been suggested for conducting systematic reviews and meta-analyses to help standardize them and improve their quality. Nonetheless, accepting the conclusions of many studies without understanding the meta-analysis can be dangerous. Therefore, this article provides an easy introduction to clinicians on performing and understanding meta-analyses.

Introduction

A systematic review collects all possible studies related to a given topic and design, and reviews and analyzes their results [ 1 ]. During the systematic review process, the quality of studies is evaluated, and a statistical meta-analysis of the study results is conducted on the basis of their quality. A meta-analysis is a valid, objective, and scientific method of analyzing and combining different results. Usually, in order to obtain more reliable results, a meta-analysis is mainly conducted on randomized controlled trials (RCTs), which have a high level of evidence [ 2 ] ( Fig. 1 ). Since 1999, various papers have presented guidelines for reporting meta-analyses of RCTs. Following the Quality of Reporting of Meta-analyses (QUORUM) statement [ 3 ], and the appearance of registers such as Cochrane Library’s Methodology Register, a large number of systematic literature reviews have been registered. In 2009, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [ 4 ] was published, and it greatly helped standardize and improve the quality of systematic reviews and meta-analyses [ 5 ].

Levels of evidence.

In anesthesiology, the importance of systematic reviews and meta-analyses has been highlighted, and they provide diagnostic and therapeutic value to various areas, including not only perioperative management but also intensive care and outpatient anesthesia [6–13]. Systematic reviews and meta-analyses include various topics, such as comparing various treatments of postoperative nausea and vomiting [ 14 , 15 ], comparing general anesthesia and regional anesthesia [ 16 – 18 ], comparing airway maintenance devices [ 8 , 19 ], comparing various methods of postoperative pain control (e.g., patient-controlled analgesia pumps, nerve block, or analgesics) [ 20 – 23 ], comparing the precision of various monitoring instruments [ 7 ], and meta-analysis of dose-response in various drugs [ 12 ].

Thus, literature reviews and meta-analyses are being conducted in diverse medical fields, and the aim of highlighting their importance is to help better extract accurate, good quality data from the flood of data being produced. However, a lack of understanding about systematic reviews and meta-analyses can lead to incorrect outcomes being derived from the review and analysis processes. If readers indiscriminately accept the results of the many meta-analyses that are published, incorrect data may be obtained. Therefore, in this review, we aim to describe the contents and methods used in systematic reviews and meta-analyses in a way that is easy to understand for future authors and readers of systematic review and meta-analysis.

Study Planning

It is easy to confuse systematic reviews and meta-analyses. A systematic review is an objective, reproducible method to find answers to a certain research question, by collecting all available studies related to that question and reviewing and analyzing their results. A meta-analysis differs from a systematic review in that it uses statistical methods on estimates from two or more different studies to form a pooled estimate [ 1 ]. Following a systematic review, if it is not possible to form a pooled estimate, it can be published as is without progressing to a meta-analysis; however, if it is possible to form a pooled estimate from the extracted data, a meta-analysis can be attempted. Systematic reviews and meta-analyses usually proceed according to the flowchart presented in Fig. 2 . We explain each of the stages below.

Flowchart illustrating a systematic review.

Formulating research questions

A systematic review attempts to gather all available empirical research by using clearly defined, systematic methods to obtain answers to a specific question. A meta-analysis is the statistical process of analyzing and combining results from several similar studies. Here, the definition of the word “similar” is not made clear, but when selecting a topic for the meta-analysis, it is essential to ensure that the different studies present data that can be combined. If the studies contain data on the same topic that can be combined, a meta-analysis can even be performed using data from only two studies. However, study selection via a systematic review is a precondition for performing a meta-analysis, and it is important to clearly define the Population, Intervention, Comparison, Outcomes (PICO) parameters that are central to evidence-based research. In addition, selection of the research topic is based on logical evidence, and it is important to select a topic that is familiar to readers without clearly confirmed the evidence [ 24 ].

Protocols and registration

In systematic reviews, prior registration of a detailed research plan is very important. In order to make the research process transparent, primary/secondary outcomes and methods are set in advance, and in the event of changes to the method, other researchers and readers are informed when, how, and why. Many studies are registered with an organization like PROSPERO ( http://www.crd.york.ac.uk/PROSPERO/ ), and the registration number is recorded when reporting the study, in order to share the protocol at the time of planning.

Defining inclusion and exclusion criteria

Information is included on the study design, patient characteristics, publication status (published or unpublished), language used, and research period. If there is a discrepancy between the number of patients included in the study and the number of patients included in the analysis, this needs to be clearly explained while describing the patient characteristics, to avoid confusing the reader.

Literature search and study selection

In order to secure proper basis for evidence-based research, it is essential to perform a broad search that includes as many studies as possible that meet the inclusion and exclusion criteria. Typically, the three bibliographic databases Medline, Embase, and Cochrane Central Register of Controlled Trials (CENTRAL) are used. In domestic studies, the Korean databases KoreaMed, KMBASE, and RISS4U may be included. Effort is required to identify not only published studies but also abstracts, ongoing studies, and studies awaiting publication. Among the studies retrieved in the search, the researchers remove duplicate studies, select studies that meet the inclusion/exclusion criteria based on the abstracts, and then make the final selection of studies based on their full text. In order to maintain transparency and objectivity throughout this process, study selection is conducted independently by at least two investigators. When there is a inconsistency in opinions, intervention is required via debate or by a third reviewer. The methods for this process also need to be planned in advance. It is essential to ensure the reproducibility of the literature selection process [ 25 ].

Quality of evidence

However, well planned the systematic review or meta-analysis is, if the quality of evidence in the studies is low, the quality of the meta-analysis decreases and incorrect results can be obtained [ 26 ]. Even when using randomized studies with a high quality of evidence, evaluating the quality of evidence precisely helps determine the strength of recommendations in the meta-analysis. One method of evaluating the quality of evidence in non-randomized studies is the Newcastle-Ottawa Scale, provided by the Ottawa Hospital Research Institute 1) . However, we are mostly focusing on meta-analyses that use randomized studies.

If the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) system ( http://www.gradeworkinggroup.org/ ) is used, the quality of evidence is evaluated on the basis of the study limitations, inaccuracies, incompleteness of outcome data, indirectness of evidence, and risk of publication bias, and this is used to determine the strength of recommendations [ 27 ]. As shown in Table 1 , the study limitations are evaluated using the “risk of bias” method proposed by Cochrane 2) . This method classifies bias in randomized studies as “low,” “high,” or “unclear” on the basis of the presence or absence of six processes (random sequence generation, allocation concealment, blinding participants or investigators, incomplete outcome data, selective reporting, and other biases) [ 28 ].

The Cochrane Collaboration’s Tool for Assessing the Risk of Bias [ 28 ]

Data extraction

Two different investigators extract data based on the objectives and form of the study; thereafter, the extracted data are reviewed. Since the size and format of each variable are different, the size and format of the outcomes are also different, and slight changes may be required when combining the data [ 29 ]. If there are differences in the size and format of the outcome variables that cause difficulties combining the data, such as the use of different evaluation instruments or different evaluation timepoints, the analysis may be limited to a systematic review. The investigators resolve differences of opinion by debate, and if they fail to reach a consensus, a third-reviewer is consulted.

Data Analysis

The aim of a meta-analysis is to derive a conclusion with increased power and accuracy than what could not be able to achieve in individual studies. Therefore, before analysis, it is crucial to evaluate the direction of effect, size of effect, homogeneity of effects among studies, and strength of evidence [ 30 ]. Thereafter, the data are reviewed qualitatively and quantitatively. If it is determined that the different research outcomes cannot be combined, all the results and characteristics of the individual studies are displayed in a table or in a descriptive form; this is referred to as a qualitative review. A meta-analysis is a quantitative review, in which the clinical effectiveness is evaluated by calculating the weighted pooled estimate for the interventions in at least two separate studies.

The pooled estimate is the outcome of the meta-analysis, and is typically explained using a forest plot ( Figs. 3 and ​ and4). 4 ). The black squares in the forest plot are the odds ratios (ORs) and 95% confidence intervals in each study. The area of the squares represents the weight reflected in the meta-analysis. The black diamond represents the OR and 95% confidence interval calculated across all the included studies. The bold vertical line represents a lack of therapeutic effect (OR = 1); if the confidence interval includes OR = 1, it means no significant difference was found between the treatment and control groups.

Forest plot analyzed by two different models using the same data. (A) Fixed-effect model. (B) Random-effect model. The figure depicts individual trials as filled squares with the relative sample size and the solid line as the 95% confidence interval of the difference. The diamond shape indicates the pooled estimate and uncertainty for the combined effect. The vertical line indicates the treatment group shows no effect (OR = 1). Moreover, if the confidence interval includes 1, then the result shows no evidence of difference between the treatment and control groups.

Forest plot representing homogeneous data.

Dichotomous variables and continuous variables

In data analysis, outcome variables can be considered broadly in terms of dichotomous variables and continuous variables. When combining data from continuous variables, the mean difference (MD) and standardized mean difference (SMD) are used ( Table 2 ).

Summary of Meta-analysis Methods Available in RevMan [ 28 ]

The MD is the absolute difference in mean values between the groups, and the SMD is the mean difference between groups divided by the standard deviation. When results are presented in the same units, the MD can be used, but when results are presented in different units, the SMD should be used. When the MD is used, the combined units must be shown. A value of “0” for the MD or SMD indicates that the effects of the new treatment method and the existing treatment method are the same. A value lower than “0” means the new treatment method is less effective than the existing method, and a value greater than “0” means the new treatment is more effective than the existing method.

When combining data for dichotomous variables, the OR, risk ratio (RR), or risk difference (RD) can be used. The RR and RD can be used for RCTs, quasi-experimental studies, or cohort studies, and the OR can be used for other case-control studies or cross-sectional studies. However, because the OR is difficult to interpret, using the RR and RD, if possible, is recommended. If the outcome variable is a dichotomous variable, it can be presented as the number needed to treat (NNT), which is the minimum number of patients who need to be treated in the intervention group, compared to the control group, for a given event to occur in at least one patient. Based on Table 3 , in an RCT, if x is the probability of the event occurring in the control group and y is the probability of the event occurring in the intervention group, then x = c/(c + d), y = a/(a + b), and the absolute risk reduction (ARR) = x − y. NNT can be obtained as the reciprocal, 1/ARR.

Calculation of the Number Needed to Treat in the Dichotomous table

Fixed-effect models and random-effect models

In order to analyze effect size, two types of models can be used: a fixed-effect model or a random-effect model. A fixed-effect model assumes that the effect of treatment is the same, and that variation between results in different studies is due to random error. Thus, a fixed-effect model can be used when the studies are considered to have the same design and methodology, or when the variability in results within a study is small, and the variance is thought to be due to random error. Three common methods are used for weighted estimation in a fixed-effect model: 1) inverse variance-weighted estimation 3) , 2) Mantel-Haenszel estimation 4) , and 3) Peto estimation 5) .

A random-effect model assumes heterogeneity between the studies being combined, and these models are used when the studies are assumed different, even if a heterogeneity test does not show a significant result. Unlike a fixed-effect model, a random-effect model assumes that the size of the effect of treatment differs among studies. Thus, differences in variation among studies are thought to be due to not only random error but also between-study variability in results. Therefore, weight does not decrease greatly for studies with a small number of patients. Among methods for weighted estimation in a random-effect model, the DerSimonian and Laird method 6) is mostly used for dichotomous variables, as the simplest method, while inverse variance-weighted estimation is used for continuous variables, as with fixed-effect models. These four methods are all used in Review Manager software (The Cochrane Collaboration, UK), and are described in a study by Deeks et al. [ 31 ] ( Table 2 ). However, when the number of studies included in the analysis is less than 10, the Hartung-Knapp-Sidik-Jonkman method 7) can better reduce the risk of type 1 error than does the DerSimonian and Laird method [ 32 ].

Fig. 3 shows the results of analyzing outcome data using a fixed-effect model (A) and a random-effect model (B). As shown in Fig. 3 , while the results from large studies are weighted more heavily in the fixed-effect model, studies are given relatively similar weights irrespective of study size in the random-effect model. Although identical data were being analyzed, as shown in Fig. 3 , the significant result in the fixed-effect model was no longer significant in the random-effect model. One representative example of the small study effect in a random-effect model is the meta-analysis by Li et al. [ 33 ]. In a large-scale study, intravenous injection of magnesium was unrelated to acute myocardial infarction, but in the random-effect model, which included numerous small studies, the small study effect resulted in an association being found between intravenous injection of magnesium and myocardial infarction. This small study effect can be controlled for by using a sensitivity analysis, which is performed to examine the contribution of each of the included studies to the final meta-analysis result. In particular, when heterogeneity is suspected in the study methods or results, by changing certain data or analytical methods, this method makes it possible to verify whether the changes affect the robustness of the results, and to examine the causes of such effects [ 34 ].

Heterogeneity

Homogeneity test is a method whether the degree of heterogeneity is greater than would be expected to occur naturally when the effect size calculated from several studies is higher than the sampling error. This makes it possible to test whether the effect size calculated from several studies is the same. Three types of homogeneity tests can be used: 1) forest plot, 2) Cochrane’s Q test (chi-squared), and 3) Higgins I 2 statistics. In the forest plot, as shown in Fig. 4 , greater overlap between the confidence intervals indicates greater homogeneity. For the Q statistic, when the P value of the chi-squared test, calculated from the forest plot in Fig. 4 , is less than 0.1, it is considered to show statistical heterogeneity and a random-effect can be used. Finally, I 2 can be used [ 35 ].

I 2 , calculated as shown above, returns a value between 0 and 100%. A value less than 25% is considered to show strong homogeneity, a value of 50% is average, and a value greater than 75% indicates strong heterogeneity.

Even when the data cannot be shown to be homogeneous, a fixed-effect model can be used, ignoring the heterogeneity, and all the study results can be presented individually, without combining them. However, in many cases, a random-effect model is applied, as described above, and a subgroup analysis or meta-regression analysis is performed to explain the heterogeneity. In a subgroup analysis, the data are divided into subgroups that are expected to be homogeneous, and these subgroups are analyzed. This needs to be planned in the predetermined protocol before starting the meta-analysis. A meta-regression analysis is similar to a normal regression analysis, except that the heterogeneity between studies is modeled. This process involves performing a regression analysis of the pooled estimate for covariance at the study level, and so it is usually not considered when the number of studies is less than 10. Here, univariate and multivariate regression analyses can both be considered.

Publication bias

Publication bias is the most common type of reporting bias in meta-analyses. This refers to the distortion of meta-analysis outcomes due to the higher likelihood of publication of statistically significant studies rather than non-significant studies. In order to test the presence or absence of publication bias, first, a funnel plot can be used ( Fig. 5 ). Studies are plotted on a scatter plot with effect size on the x-axis and precision or total sample size on the y-axis. If the points form an upside-down funnel shape, with a broad base that narrows towards the top of the plot, this indicates the absence of a publication bias ( Fig. 5A ) [ 29 , 36 ]. On the other hand, if the plot shows an asymmetric shape, with no points on one side of the graph, then publication bias can be suspected ( Fig. 5B ). Second, to test publication bias statistically, Begg and Mazumdar’s rank correlation test 8) [ 37 ] or Egger’s test 9) [ 29 ] can be used. If publication bias is detected, the trim-and-fill method 10) can be used to correct the bias [ 38 ]. Fig. 6 displays results that show publication bias in Egger’s test, which has then been corrected using the trim-and-fill method using Comprehensive Meta-Analysis software (Biostat, USA).

Funnel plot showing the effect size on the x-axis and sample size on the y-axis as a scatter plot. (A) Funnel plot without publication bias. The individual plots are broader at the bottom and narrower at the top. (B) Funnel plot with publication bias. The individual plots are located asymmetrically.

Funnel plot adjusted using the trim-and-fill method. White circles: comparisons included. Black circles: inputted comparisons using the trim-and-fill method. White diamond: pooled observed log risk ratio. Black diamond: pooled inputted log risk ratio.

Result Presentation

When reporting the results of a systematic review or meta-analysis, the analytical content and methods should be described in detail. First, a flowchart is displayed with the literature search and selection process according to the inclusion/exclusion criteria. Second, a table is shown with the characteristics of the included studies. A table should also be included with information related to the quality of evidence, such as GRADE ( Table 4 ). Third, the results of data analysis are shown in a forest plot and funnel plot. Fourth, if the results use dichotomous data, the NNT values can be reported, as described above.

The GRADE Evidence Quality for Each Outcome

N: number of studies, ROB: risk of bias, PON: postoperative nausea, POV: postoperative vomiting, PONV: postoperative nausea and vomiting, CI: confidence interval, RR: risk ratio, AR: absolute risk.

When Review Manager software (The Cochrane Collaboration, UK) is used for the analysis, two types of P values are given. The first is the P value from the z-test, which tests the null hypothesis that the intervention has no effect. The second P value is from the chi-squared test, which tests the null hypothesis for a lack of heterogeneity. The statistical result for the intervention effect, which is generally considered the most important result in meta-analyses, is the z-test P value.

A common mistake when reporting results is, given a z-test P value greater than 0.05, to say there was “no statistical significance” or “no difference.” When evaluating statistical significance in a meta-analysis, a P value lower than 0.05 can be explained as “a significant difference in the effects of the two treatment methods.” However, the P value may appear non-significant whether or not there is a difference between the two treatment methods. In such a situation, it is better to announce “there was no strong evidence for an effect,” and to present the P value and confidence intervals. Another common mistake is to think that a smaller P value is indicative of a more significant effect. In meta-analyses of large-scale studies, the P value is more greatly affected by the number of studies and patients included, rather than by the significance of the results; therefore, care should be taken when interpreting the results of a meta-analysis.

When performing a systematic literature review or meta-analysis, if the quality of studies is not properly evaluated or if proper methodology is not strictly applied, the results can be biased and the outcomes can be incorrect. However, when systematic reviews and meta-analyses are properly implemented, they can yield powerful results that could usually only be achieved using large-scale RCTs, which are difficult to perform in individual studies. As our understanding of evidence-based medicine increases and its importance is better appreciated, the number of systematic reviews and meta-analyses will keep increasing. However, indiscriminate acceptance of the results of all these meta-analyses can be dangerous, and hence, we recommend that their results be received critically on the basis of a more accurate understanding.

1) http://www.ohri.ca .

2) http://methods.cochrane.org/bias/assessing-risk-bias-included-studies .

3) The inverse variance-weighted estimation method is useful if the number of studies is small with large sample sizes.

4) The Mantel-Haenszel estimation method is useful if the number of studies is large with small sample sizes.

5) The Peto estimation method is useful if the event rate is low or one of the two groups shows zero incidence.

6) The most popular and simplest statistical method used in Review Manager and Comprehensive Meta-analysis software.

7) Alternative random-effect model meta-analysis that has more adequate error rates than does the common DerSimonian and Laird method, especially when the number of studies is small. However, even with the Hartung-Knapp-Sidik-Jonkman method, when there are less than five studies with very unequal sizes, extra caution is needed.

8) The Begg and Mazumdar rank correlation test uses the correlation between the ranks of effect sizes and the ranks of their variances [ 37 ].

9) The degree of funnel plot asymmetry as measured by the intercept from the regression of standard normal deviates against precision [ 29 ].

10) If there are more small studies on one side, we expect the suppression of studies on the other side. Trimming yields the adjusted effect size and reduces the variance of the effects by adding the original studies back into the analysis as a mirror image of each study.