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Evidence Based Literature Searching: Types of Studies

Why is the type of study important?

Thinking about the type of study will help you build a good clinical question. As you go up the pyramid (see Strength of the Evidence Pyamid below) the quality of the references goes up but the amount of references found goes down.


  • Background/Expert Opinion-Evidence in these resources may vary from expert opinion to high levels of evidence


  • Case Series/ Case Reports-consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have little statistical validity.

    Case Controlled Studies- A case control study starts with patients who already have the outcome and looks backwards to possible exposures.


  • Cohort Studies-A cohort study starts with the exposure and follows patients forward to an outcome.


  • Randomized Controlled Trials (RCTs)- A RCT randomly assigns the exposures and then follows patients forward to an outcome.


  • Critically Appraised Individual Articles [Article Synopsis]-Authors of critically-appraised individual articles evaluate and synopsize individual research studies.


  • Critically Appraised Topics [Evidence Synthesis]-Authors of critically-appraised topics evaluate and synthesize multiple research studies.


  • Systematic Reviews-Authors of a systematic review ask a specific clinical question, perform a comprehensive literature search, eliminate the poorly done studies and attempt to make practice recommendations based on the well-done studies.

Strength of the Evidence Pyramid


Strength of Evidence Pyramid

Based on the EBM Pyramid. Copyright © 2006–2011. Trustees of Dartmouth College and Yale University. All Rights Reserved. Produced by Jan Glover, David Izzo, Karen Odato and Lei Wang.

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