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Evidence-Based Public Health: Sources of Evidence

Hierarchy or Levels of Evidence Pyramid

Not all evidence is judged to be of equal value; that is, there are hierarchies of research design that are evaluated to have different strengths or different levels of value in the decision making process.  Of course, the best are at the top and as the pyramids indicate, you have much fewer resources at the top than at the bottom of the pyramids. [Note: The TRIP - or Turning Research Into Practice - Database is freely available at]

Examples of Levels of Evidence

When you're determining which category an article falls under, there are a few things you should consider.  This document will offer tips for assessing the level of evidence in PubMed and provide several examples.

Try these guides for more help:

Hierarchy of Evidence Definitions

  • Systematic review: A review of a body of data that uses explicit methods to locate primary studies, and explicit criteria to assess their quality.
  • Meta-analysis: A statistical analysis that combines or integrates the results of several independent clinical trials considered by the analyst to be "combinable" usually to the level of re-analyzing the original data; also sometimes called pooling or quantitative synthesis.
    • Both Systematic review and meta-analysis are sometimes called "overviews".
  • Randomized Controlled Trials: Individuals are randomly allocated to a control group and a group who receive a specific intervention. Otherwise the two groups are identical for any significant variables. They are followed up for specific end points.
  • Cohort Studies: Groups of people are selected on the basis of their exposure to a particular agent and followed up for specific outcomes.
  • Case Series: "Cases" with the condition are matched with "controls" without, and a retrospective analysis is used to look for differences between the two groups.
  • Case Study: A report based on a single patient or subject; sometimes collected together into a short series.
  • Background lnformation/Expert Opinion: Fully referenced expert topic reviews written by recognized authorities who review the topic, synthesize the evidence, summarize key findings, and provide specific recommendations.
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