A HIERARCHY OF EVIDENCE WITHIN THE SCHOLARLY LITERATURE

Shelly 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 toKnow 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

Animal Research

Case Series/Case Reports

Randomized Controlled Trials

Cohort Studies

Systematic Reviews

Case Control Studies

Meta Analysis

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 -3Systematic 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.

Use the Right Type of Study for Your Question

Type of Question

Suggested best type of study

Diagnosis prospective, blind comparison to a gold standard

Therapy RCT > cohort > case control > case series

Prognosis cohort study > case control > case series

Harm/Etiology

RCT > cohort > case control > case series

Prevention RCT > cohort study > case control > case series

Clinical Exam

prospective, blind comparison to gold standard

Cost Benefit economic analysis

MOVE ON TO MODULE 5 – CONTROLLED VOCABULARIES