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Learn about the different types of studies in the evidence hierarchy and how to select the correct study type to answer research questions.
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A HIERARCHY OF EVIDENCE WITHIN THE SCHOLARLY LITERATUREShelly Warwick, Ph.D. 2012 – Permission is granted to reproduce and edit this work for non-commercial educational use as long as attribution is provided and the edited work is also available under the same terms of license.
After this Unit You Should Be Able to • Know where each type of study falls within the evidence hierarchy. • Explain the difference between a narrative review, systematic review and meta-analysis. • Select the correct study type(s) to answer a specific research question.
Remember the Evidence Hierarchy The top of the pyramid is best – but if high level studies aren’t available – use what is Meta Analysis Systematic Reviews Randomized Controlled Trials Cohort Studies Case Control Studies Case Series/Case Reports Animal Research
Differences in Study Types in the Pyramid -1 • Animal Studies – studies using animals not humans • Case Series and Case Reports – reports of the treatment of individual patients or a single patient. No use of control group. No statistical validity. • Case Control Studies – retrospective studies comparing patients with a specific condition or outcome to those without the condition or outcome to determine exposures or causes. Can show statistical relationship but not necessarily a causal relationship.
Differences in Study Types in the Pyramid -2 . • Cohort Studies – prospective observational studies of patients already being treated or who have had exposure. Studied group is often compared to general population or similar population not treated. • Randomized Controlled Clinical Trials (RCT) – treatment of real patients under controlled conditions – including randomization to treatment or no treatment and blinding of which group the individual is assigned from both the patient and individual(s) providing follow-up and collecting results.
Differences in Study Types in the Pyramid -3 • Systematic Reviews - based on extensive exhaustive literature search to identify studies that focus on a highly defined question. Results summarized according to predetermined criteria. Not to be confused with Narrative Review articles that review recent literature on a topic but are usually not exhaustive. • Meta-analysis – goes beyond the Systematic Review to combining results of studies using accepted statistical methodology to produce a single conclusion.
