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Evidence Synthesis

This guide provides an introduction to evidence synthesis as it is understood in evidence-based practice. clinical research and healthcare policy arenas.

Evidence Synthesis Examples

There are different methods to synthesize evidence depending on the type of study (randomized controlled trial, ethnography), the type of data collected (quantitative or qualitative).  The follow three sections provide a list and brief summaries of evidence synthesis methods based on these criteria.  The third subsection provides a listing of theoretical method typologies that can be applied to different types of synthesis.

In this page, you will find resources and information about types of studies that use evidence synthesis as a step in the review process (systematic reviews) as well as types of studies that can be used to develop an evidence synthesis (primary literature).

Synthesized Literature

Synthesized literature aims to collect the evidence from multiple primary studies and synthesize the results in order to make recommendations for practice.  When well done, these types of articles are often considered to be a high level of evidence.  The following are common types of synthesized literature:

Systematic Reviews
  • Attempt to use only the most rigorously conducted studies to make recommendations for practice.
  • Typically involve 1) asking a specific clinical question, 2) performing a comprehensive search, 3) using pre-determined inclusion/exclusion criteria, and 4) pooling information to answer the question.
Meta-Analyses
  • Takes the systematic review process a step further by analyzing all data from the studies using sophisticated statistics to then make recommendations for practice.
Evidence-Based Clinical Practice Guideline
  • A set of principles developed by government agencies, professional societies, or expert panels that are meant to guide practitioners with patient care decisions.
  • The quality of these can vary, so pay attention to the methods employed in their construction and the levels of evidence used for the recommendations contained within.

The following resources are a good place to start when looking for synthesized literature:

Primary Literature

If you cannot locate any high-level syntheses of evidence, you can try searching the primary literature for research articles.  Remember to consider what type of study is being presented and its methods when selecting primary literature.  See Appraise the Evidence for more information about different study types.  The following are some common study designs that you may see in the primary literature:

Randomized Controlled Trial
  • Study in which two groups are randomized to either receive an intervention/treatment, or be the control group (i.e. receive a comparison intervention/treatment or a placebo).
  • Process is designed to minimize the possibility of bias and confounding factors.
  • Experimental study design.
Cohort Study
  • Follows a group/cohort prospectively for a period of time to look for an association between risk factors and outcomes.
  • Can be performed retrospectively as well.
  • Observational study design.
Case-Control Study
  • Compares two groups (one with and one without the outcome of interest) and looks back retrospectively to determine if an earlier risk factor or exposure is associated with the outcome.
  • Observational study design.

The following resources are a good place to start when looking for primary literature; however, you always want to consider your subject as well.  If you are researching an inter-disciplinary topic (e.g. psychological effects of a disease, effective patient education methods), you may want to consider looking in another subject-specific database as well.