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Evidence Synthesis: Evidence Synthesis Overview

How Librarians Can Help

At Northwestern University Libraries, we offer general consultations regarding evidence synthesis projects, including literature reviews and systematic reviews. This includes:

  • a basic overview of the systematic review process 
  • guidance on developing a search strategy and selecting databases
  • methods for collecting and organizing articles
  • resource recommendations for analyzing results

If the project requires more long-term support, let us know. We may be able to provide additional assistance such as developing and reviewing search strings. Depending on the level of support provided, the researcher may need to provide acknowledgment of the librarian in the final publication.

If you'd like to set up a consultation, complete the Evidence Synthesis Consultation Request form.

Attribution

Unless otherwise noted, this guide was adapted from Cornell University's "A Guide to Evidence Synthesis"

What is Evidence Synthesis?

Evidence synthesis refers to any method of identifying, selecting, and combining results from multiple studies. Systematic reviews and literature reviews are two methods of identifying and providing summaries of existing literature on a particular topic. They are quite different in scope and practice.

  Traditional Literature Review Systematic Review
Definition Qualitatively summarizes evidence on a topic using informal or subjective methods to collect and interpret studies. Methodically and systematically identifies all literature on a well-formulated research topic, including published and unpublished studies.
Goals To provide a summary or overview of a topic To provide evidence for practice and policy-making, and to identify gaps in research
Question Topics may be broad in scope; the goal of the review may be to place one's own research within the existing body of knowledge, or to gather information that supports a particular viewpoint. Starts with a well-defined research question to be answered by the review. Reviews are conducted with the aim of finding all existing evidence in an unbiased, transparent, and reproducible way.
Timeline Weeks or months 12-18 months
Synthesis of existing research Conclusions are more qualitative and may not be based on study quality. Bases conclusion on quality of the studies and provide recommendations for practice or to address knowledge gaps.

Adapted from Kysh, L. (2013). What’s in a name? The difference between a systematic review and a literature review and why it matters. [Poster].Retrieved from http://dx.doi.org/10.6084/m9.figshare.766364. Licensed under CC-BY. 

Types of Evidence Synthesis

Types of evidence synthesis include:

Systematic Review

  • Systematically and transparently collect and categorize existing evidence on a broad question of scientific, policy or management importance.
  • Compares, evaluates, and synthesizes evidence in a search for the effect of an intervention. 
  • Time-intensive and often take months to a year or more to complete. 
  • The most commonly referred to type of evidence synthesis. Sometimes confused as a blanket term for other types of reviews.

​​Literature (Narrative) Review

  • A broad term referring to reviews with a wide scope and non-standardized methodology. 
  • Search strategies, comprehensiveness, and time range covered will vary and do not follow an established protocol.

Scoping Review or Evidence Map

  • Systematically and transparently collect and categorize existing evidence on a broad question of scientific, policy or management importance.
  • Seeks to identify research gaps and opportunities for evidence synthesis rather than searching for the effect of an intervention. 
  • May critically evaluate existing evidence, but does not attempt to synthesize the results in the way a systematic review would. (see EE Journal and CIFOR)
  • May take longer than a systematic review.
  • See Arksey and O'Malley (2005) for methodological guidance.

​Rapid Review

  • Applies Systematic Review methodology within a time-constrained setting.
  • Employs methodological "shortcuts" (limiting search terms for example) at the risk of introducing bias.
  • Useful for addressing issues needing quick decisions, such as developing policy recommendations.
  • See Evidence Summaries: The Evolution of a Rapid Review Approach

Umbrella Review

  • Reviews other systematic reviews on a topic. 
  • Often defines a broader question than is typical of a traditional systematic review.
  • Most useful when there are competing interventions to consider.

Meta-analysis

  • Statistical technique for combining the findings from disparate quantitative studies.
  • Uses statistical methods to objectively evaluate, synthesize, and summarize results.
  • May be conducted independently or as part of a systematic review.

Which type of review is right for you?