Systematic Reviews: Knowledge Synthesis Tools & Types

This guide provides information and resources which may be helpful when undertaking a systematic review or other type of knowledge synthesis.

Select the Appropriate Review Methodology

"Decisions" icon by Adrien Coquet from the Noun Project

Many review methodologies exist. This guide focuses on quantitative review methodologies commonly published in the field of medicine and health care. Be aware that other knowledge synthesis frameworks exist beyond the typologies described here.

The Review Family

Systematic reviews synthesize existing research, “enabling decisions on effectiveness to be based on all relevant studies of adequate quality.”Finding What Works in Health Care: Standards for Systematic Reviews, defines a systematic reviews as a “scientific investigation that focuses on a specific question and uses explicit, prespecified scientific methods to identify, select, assess, and summarize the findings of similar but separate studies.”Systematic reviews usually take between 9-12 months to complete.

Following a systematic review, a meta-analysis of the data from the individual studies included in the SR may be performed.1 “Meta-analysis is a quantitative, formal, epidemiological study design used to systematically assess the results of previous research to derive conclusions about that body of research.”3 In other words, meta-analysis “statistically pools the results of the individual studies to produce a single estimate of the effect.”4 A systematic review and meta-analysis can take between 10-12 months to complete.

Systematic reviews require narrow research questions, while scoping reviews “take into account a broad range of research and wider conceptual issues…”5 Scoping reviews gather knowledge on “an exploratory research question aimed at mapping key concepts, types of evidence, and gaps in research related to a defined area or filed by systematically searching, selecting, and synthesizing existing knowledge.”6 Frequently scoping reviews are used for “reconnaissance,” to “clarify working definitions and conceptual boundaries of a topic or field.”5 Due to the broad nature of scoping review questions, large quantities of literature may need to be reviewed. Scoping reviews often take at least 12 months to complete.

Conducting and adhering to the standards of high quality systematic reviews can be time consuming.7 The mean time for a systematic review project is 67.3 weeks.8 Rapid reviews may be useful to synthesize current evidence and enable evidence-based decision making in the midst of emergent medical situations, e.g., the Ebola epidemic, avian influenza, and the COVID-19 pandemic. Rapid reviews require transparency and systematic methodologies7, but “[c]urtailing the duration of the review process runs the risk of introducing bias.”9 The rapid review process generally takes 1-6 months.

A review that compiles "evidence from multiple reviews into one accessible and usable document."9 This type of review "focuses on a broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results." 9 In short, an umbrella review is a systematic review of systematic reviews.

Narrative reviews, or non-systematic literature reviews, are a method of knowledge synthesis, but do not have the stringent methodologies or reporting standards of systematic-type reviews. Narrative reviews do not require a clinically focused research question and inclusion/exclusion criteria are not usually explicitly described. Because a systematic search of the literature is not required for this type of review, a methodology section is not mandatory. Included studies are not evaluated for quality and no risk of bias is performed. Narrative reviews may address more than a single question and are useful for addressing general debates, identifying current lack of knowledge, providing rationales for future research, or speculating on types of interventions available.10

References & Recommended Reading

1.         Crombie IK, Davis HT. What is meta-analysis? 2009.

2.         Eden J, Levit L, Berg A. Finding What Works in Health Care : Standards for Systematic Reviews. National Academies Press; 2011. Accessed May 5, 2021.

3.         Haidich AB. Meta-analysis in medical research. Hippokratia. 2010;14(Suppl 1):29-37.

4.         Umscheid CA. A Primer on Performing Systematic Reviews and Meta-analyses. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 2013;57(5):725-734.

5.         Cooper S, Cant R, Kelly M, et al. An Evidence-Based Checklist for Improving Scoping Review Quality. Clin Nurs Res. 2019.

6.         Colquhoun HL, Levac D, O'Brien KK, et al. Scoping reviews: Time for clarity in definition, methods, and reporting. J Clin Epidemiol. 2014;67(12):1291-1294.

7.          Schünemann HJ, Moja L. Reviews: Rapid! Rapid! Rapid!.and systematic. Syst Rev. 2015;4(1).

8.          Borah R, Brown AW, Capers PL, Kaiser KA. Analysis of the time and workers needed to conduct systematic reviews of medical interventions using data from the PROSPERO registry. BMJ open. 2017;7(2):e012545.

9.          Grant MJ, Booth A. A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal. 2009;26(2):91-108.

10.        Ferrari R. Writing narrative style literature reviews.  Medical Writing. 2015;24(4):230-235. doi:10.1179/2047480615Z.000000000329