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Answering Clinical Questions: Hierarchy of Evidence

Clinical evidence based practice resources

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The Hierarchy of Evidence

As you move up the pyramid, the amount of literature decreases but its's clinical relevance increases.

Pyramid of the hierarchy of evidence.

What is the Hierarchy of Evidence?

The pyramid above organises medical information into a hierarchy of evidence.

Resources with the highest levels of critical appraisal and synthesis are placed at the top of the pyramid. These resources are generally called tertiary resources and present a summary and analysis of primary and secondary information found lower in the pyramid.  Information from resources higher in the pyramid is quicker to access and read because the appraisal is done for you.

Hierarchy of Single Studies

The lowest level of the hierarchy of evidence pyramid, Single Case Studies, can be further broken down. The pyramid below shows the levels of syntheses of one or more single studies where the ranking of the single study types themselves as evidence.

Pyramid of single studies research types.

Definitions of Levels of Evidence

Decision Support Systems provide management guidance by linking individual patient information to the best available evidence from relevant research.

Evidence Based Summaries draw on systematic reviews or the best available primary studies to integrate the highest level evidence available.

Evidence Based Guidelines use evidence drawn from systematic reviews and studies. They are problem focused and may take into account resources and practice relevant to the organisation sponsoring the guideline development.

Systematic Reviews use a transparent process to identify, appraise and summarise studies in relation to a defined clinical question.  A systematic review may or may not include a meta-analysis - a summary of the medical literature that used statistical techniques to combine study results.  Ideally a meta-analysis should be done in the context of a systematic review.

Synopses of Single Studies critically appraise individual articles (primary studies) and include a summary of key methods and findings of the paper and a critique or commentary related to clinical practice.

Single Studies are primary research including:

  • Randomised Control Trials (RCTs)
  • Cohort studies
  • Case-controlled studies
  • Case series/case reports
  • Qualitative studies


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