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Systematic Reviews

All about systematic reviews: what they are, their history, types, how to read them, how to conduct them, and how to get help with them from the library.

Who is this guide for and what can be found in it?

This guide aims to support all OHSU members' systematic review education and activities, orienting OHSU members who are new to systematic reviews and facilitating the quality, rigor, and reproducibility of systematic reviews produced by OHSU members.

In it you will find:

What are systematic reviews?

"A systematic review is a summary of the medical literature that uses explicit and reproducible methods to systematically search, critically appraise, and synthesize on a specific issue. It synthesizes the results of multiple primary studies related to each other by using strategies that reduce biases and random errors."

Gopalakrishnan S, Ganeshkumar P. Systematic Reviews and Meta-analysis: Understanding the Best Evidence in Primary Healthcare. J Family Med Prim Care. 2013;2(1):9-14. doi:10.4103/2249-4863.109934

Systematic Reviews are a vital resource used in the pursuit of Evidence-Based Practice (EBP):

  • These studies can be found near the top of the Evidence Pyramid, which ranks sources of information and study designs by the level of evidence contained within them
  • This ranking is based on the level of scientific rigor employed in their methods and the quality and reliability of the evidence contained within these sources
  • A higher ranking means that we can be more confident that their conclusions are accurate and have taken measures to limit bias

Evidence-Based Medicine (EBM) Pyramid, visualizing how the quality of evidence strengthens as you go up the pyramid from Background Information/Expert Opinion at the bottom of the pyramid to a group of three unfiltered information sources: Case-Controlled Studies and Case Series/Reports, Cohort Studies, and Randomized Controlled Trials (RCTs), and then to a group of filtered information sources: Critically-Appraised Individual Articles (aka Article Synopses), Critically-Appraised Topics (aka Evidence Syntheses), and Systematic Reviews, which are at the top of the pyramid.

Research design and evidence, by CFCF, CC BY-SA 4.0, via Wikimedia Commons

Things to know about systematic reviews:

  • Systematic reviews are a type of research study
  • Systematic reviews aim to provide a comprehensive and unbiased summary of the existing evidence on a particular research question
  • There are many types of systematic reviews, each designed to address a specific type of research purpose and with their own strengths and weaknesses
  • The choice of what type of review to produce typically will depend on the nature of the research question and the resources that are available on the topic

The practice of producing systematic reviews is sometimes referred to by other names such as:

  • Evidence Synthesis
  • Knowledge Synthesis
  • Research Synthesis

This guide tries to stick with the term "Systematic Reviews" unless a specific type of systematic review is being discussed.

How do systematic reviews differ from narrative literature reviews?

While all reviews combat information overload in the health sciences by summarizing the literature on a topic, different types of reviews have different approaches. The term systematic review is often conflated with narrative literature reviews, which can lead to confusion and misunderstandings when seeking help with conducting them. This table helps clarify the differences.

Comparing Systematic Reviews to Narrative Literature Reviews
  Systematic Reviews Narrative Literature Reviews
Authors Two or more authors are involved in good quality systematic reviews, many comprise multiple authors with expertise in the different stages of the review One or more authors, usually experts in the topic of interest
Study Protocol Written study protocol which includes details of the methods to be used No study protocol
Research Question

Specific question which may have all or some of PICOTS components (Population, Intervention, Comparator, Outcome, Time, and Setting)

Hypothesis is clearly stated

Broad to specific question, hypothesis typically not stated
Search Strategy Detailed and comprehensive search strategy is developed, preferably utilizing the skills of an experienced librarian No detailed search strategy, supporting literature search is probably conducted using general keyword queries
Sources of Literature

List of databases, websites and other sources of included studies are listed

Both published and unpublished literature are considered

Not usually stated and non-exhaustive, featured articles often already known to the authors

Prone to publication bias

Selection Criteria Specific inclusion and exclusion criteria for evaluating abstracts and articles are described No specific selection criteria, usually subjective
Highly prone to selection bias
Critical Appraisal Rigorous appraisal of study quality and methodology Variable evaluation of study quality or method
Synthesis Narrative, quantitative, and/or qualitative synthesis techniques are used Often only qualitative synthesis of evidence
Conclusions Conclusions drawn are evidence based Sometimes evidence based, but can be significantly influenced by author’s personal beliefs and biases and its conclusions may be highly subjective
Reproducibility Accurate documentation of methods means process can be repeated Undocumented or incomplete methodology means findings cannot be reproduced independently
Updates Systematic reviews can be periodically updated to include new evidence Cannot be accurately updated