The first step in any systematic review or other type of evidence synthesis project is to search the existing literature to identify what research, both primary and secondary, has already been conducted. As with any publication, your review will likely need to be original/novel in order to be of interest to editors and publications. In addition, duplicating a previously done study may not add new understandings to the body of evidence. Here are some questions for consideration:
All of these questions will help you identify why you would conduct this new study. It is disheartening (at best) to learn part way through an evidence synthesis project that the research has already been conducted.
Search for published studies that address your research question. This should be done in several different databases to ensure you have a solid sense of what has already been accomplished. It will also inform the way that you create your search strategy for this study, as you'll learn the types of words that are used to describe this research question, publications that have published these types of studies, and how the articles have been indexed within the databases. Below are a few databases that you might consider searching for health sciences publications.
In addition to finding articles that have already been published, you will need to search registries to see if others are currently in the process of researching this topic/question, just as you would for clinical research. Here are a few repositories for you to search:
While considering conducting a literature review, you should compare your draft research question to the different review types that can be used to explore the existing research. Some types, such as narrative reviews, do not consider the literature search as a formal methodology, while others such as Systematic Reviews view the literature search as a reproducible research methodology.
As a first step, search the literature for other published articles and studies that address the same or similar research question. The quantity, quality, and depth of existing research will be an important component in deciding on a review type.
Selection of the review type includes not only aspirations for what the review could accomplish, but also pragmatic limitations based on how much time the team has to devote to the project, how many team members are participating in the review, and deadlines for the review completion (such as the date of an upcoming conference). In addition, the breadth of the research question may result in a large number of search results; this should be considered in terms of the number of team members involved in the screening and abstraction of the included studies, as well as whether the research question should be narrowed or include more limitations/exclusion criteria in order to satisfy the practical limitations of the team.
Use the decision tree below from Cornell University to determine what type of review best suits your question/topic and available resources. The PDF is linked for you to view/download along with descriptions of the review types, with an image of the decision tree displayed on this page. To learn more about the different types, purposes, and methods of reviews, click the "Types of Reviews" link below the decision tree.
Reproduced from Grant, M. J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26: 91–108. doi:10.1111/j.1471-1842.2009.00848.x
Label | Description | Search | Appraisal | Synthesis | Analysis |
---|---|---|---|---|---|
Critical Review | Aims to demonstrate writer has extensively researched literature and critically evaluated its quality. Goes beyond mere description to include degree of analysis and conceptual innovation. Typically results in hypothesis or model | Seeks to identify most significant items in the field | No formal quality assessment. Attempts to evaluate according to contribution | Typically narrative, perhaps conceptual or chronological | Significant component: seeks to identify conceptual contribution to embody existing or derive new theory |
Literature Review | Generic term: published materials that provide examination of recent or current literature. Can cover wide range of subjects at various levels of completeness and comprehensiveness. May include research findings | May or may not include comprehensive searching | May or may not include quality assessment | Typically narrative | Analysis may be chronological, conceptual, thematic, etc. |
Mapping Review / Systematic Map | Map out and categorize existing literature from which to commission further reviews and/or primary research by identifying gaps in research literature | Completeness of searching determined by time/scope constraints | No formal quality assessment | May be graphical and tabular | Characterizes quantity and quality of literature, perhaps by study design and other key features. May identify need for primary or secondary research |
Meta-Analysis | Technique that statistically combines the results of quantitative studies to provide a more precise effect of the results | Aims for exhaustive, comprehensive searching. May use funnel plot to assess completeness | Quality assessment may determine inclusion/exclusion and/or sensitivity analyses | Graphical and tabular with narrative commentary | Numerical analysis of measures of effect assuming absence of heterogeneity |
Mixed Studies Review / Mixed Methods Review | Refers to any combination of methods where one significant component is a literature review (usually systematic). Within a review context it refers to a combination of review approaches for example combining quantitative with qualitative research or outcome with process studies | Requires either very sensitive search to retrieve all studies or separately conceived quantitative and qualitative strategies | Requires either a generic appraisal instrument or separate appraisal processes with corresponding checklists | Typically both components will be presented as narrative and in tables. May also employ graphical means of integrating quantitative and qualitative studies | Analysis may characterise both literatures and look for correlations between characteristics or use gap analysis to identify aspects absent in one literature but missing in the other |
Overview | Generic term: summary of the [medical] literature that attempts to survey the literature and describe its characteristics | May or may not include comprehensive searching (depends whether systematic overview or not) | May or may not include quality assessment (depends whether systematic overview or not) | Synthesis depends on whether systematic or not. Typically narrative but may include tabular features | Analysis may be chronological, conceptual, thematic, etc. |
Qualitative Systematic Review / Qualitative Evidence Synthesis | Method for integrating or comparing the findings from qualitative studies. It looks for ‘themes’ or ‘constructs’ that lie in or across individual qualitative studies | May employ selective or purposive sampling | Quality assessment typically used to mediate messages not for inclusion/exclusion | Qualitative, narrative synthesis | Thematic analysis, may include conceptual models |
Rapid Review | Assessment of what is already known about a policy or practice issue, by using systematic review methods to search and critically appraise existing research | Completeness of searching determined by time constraints | Time-limited formal quality assessment | Typically narrative and tabular | Quantities of literature and overall quality/direction of effect of literature |
Scoping Review | Preliminary assessment of potential size and scope of available research literature. Aims to identify nature and extent of research evidence (usually including ongoing research) | Completeness of searching determined by time/scope constraints. May include research in progress | No formal quality assessment | Typically tabular with some narrative commentary | Characterizes quantity and quality of literature, perhaps by study design and other key features. Attempts to specify a viable review |
State-of-the-Art Review | Tend to address more current matters in contrast to other combined retrospective and current approaches. May offer new perspectives on issue or point out area for further research | Aims for comprehensive searching of current literature | Aims for comprehensive searching of current literature | Typically narrative, may have tabular accompaniment | Current state of knowledge and priorities for future investigation and research |
Systematic Review | Seeks to systematically search for, appraise and synthesis research evidence, often adhering to guidelines on the conduct of a review | Aims for exhaustive, comprehensive searching | Quality assessment may determine inclusion/exclusion | Typically narrative with tabular accompaniment | What is known; recommendations for practice. What remains unknown; uncertainty around findings, recommendations for future research |
Systematic Search and Review | Combines strengths of critical review with a comprehensive search process. Typically addresses broad questions to produce ‘best evidence synthesis’ | Aims for exhaustive, comprehensive searching | May or may not include quality assessment | Minimal narrative, tabular summary of studies | What is known; recommendations for practice. Limitations |
Systematized Review | Attempt to include elements of systematic review process while stopping short of systematic review. Typically conducted as postgraduate student assignment | May or may not include comprehensive searching | May or may not include quality assessment | Typically narrative with tabular accompaniment | What is known; uncertainty around findings; limitations of methodology |
Umbrella Review | Specifically refers to review compiling evidence from multiple reviews into one accessible and usable document. Focuses on broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results | Identification of component reviews, but no search for primary studies | Quality assessment of studies within component reviews and/or of reviews themselves | Graphical and tabular with narrative commentary | What is known; recommendations for practice. What remains unknown; recommendations for future research |
When developing a searchable question, it helps to identify the key concepts of your research proposal. A clear and precise search question can be used to develop search terms during the literature searching process.
There are a number of frameworks available to use to help you break your question into its key concepts. Take a look at the frameworks below.
From BMJ Best Practice:
The PICO (Population, Intervention, Comparator and Outcomes) model captures the key elements and is a good strategy to provide answerable questions.
Population: who are the relevant patients or the target audience for the problem being addressed?
Example: In women with non-tubal infertility
Intervention: what intervention is being considered?
Example: …would intrauterine insemination…
Comparator: what is the main comparator to the intervention that you want to assess?
Example: …when compared with fallopian tube sperm perfusion…
Outcomes: what are the consequences of the interventions for the patient? Or what are the main outcomes of interest to the patient or decision maker?
Example: …lead to higher live birth rates with no increase in multiple pregnancy, miscarriage or ectopic pregnancy rates?
How to clarify a clinical question. (n.d.). BMJ Best Practice. Retrieved October 26, 2022, from https://bestpractice.bmj.com/info/us/toolkit/learn-ebm/how-to-clarify-a-clinical-question/
From "Formulating the Evidence Based Practice Question":
Setting: What is the context for the question? The research evidence should reflect the context or the research findings may not be transferable.
Perspective: Who are the users, potential users, or stakeholders of the service?
Intervention: What is being done for the users, potential users, or stakeholders?
Comparison: What are the alternatives? An alternative might maintain the status quo and change nothing.
Evaluation: What measurement will determine the intervention’s success? In other words, what is the result?
Davies, K. S. (2011). Formulating the Evidence Based Practice Question: A Review of the Frameworks. Evidence Based Library and Information Practice, 6(2), Article 2. https://doi.org/10.18438/B8WS5N
From "How CLIP became ECLIPSE":
Expectation—what does the search requester want the information for (the original ‘I’s)?
Client Group
Location
Impact: what is the change in the service, if any, which is being looked for? What would constitute success? How is this being measured?
Professionals
Service: for which service are you looking for information? For example, outpatient services, nurse-led clinics, intermediate care
Wildridge, V., & Bell, L. (2002). How CLIP became ECLIPSE: A mnemonic to assist in searching for health policy/management information. Health Information & Libraries Journal, 19(2), 113–115. https://doi.org/10.1046/j.1471-1842.2002.00378.x