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THL Evidence-Based Practice

Guides, Tutorials and Presentations offered by Taubman Health Sciences Library on Evidence-Based Practice

Study Design Pyramid

Pyramids are often used to visually represent the quality of evidence provided by different study designs used in research. The specific study designs included can differ by discipline and purpose. Most study design pyramids have meta analyses and systematic reviews at the top to indicate they are the highest levels of evidence. 

Level of Evidence Pyramid with two types of secondary literature at the top of the pyramid (that is, systematic review and meta analysis) and four primary literature study designs at the bottom of the pyramid. From top to bottom, the primary literature study designs are randomized controlled trial, cohort study, case control study, and case series or case report. A line separates the primary and secondary literature.

Generally, level of evidence hierarchies assume the studies were conducted according to the best practices for the particular study design. This means a well-conducted, rigorous cohort study could provide better quality evidence than a poorly conducted randomized controlled trial. This is one reason Appraise is an important step in the Evidence-Based Practice process.

Study Designs

Primary vs Secondary Literature

Primary (unfiltered) evidence: Includes original individual studies, such as controlled trials, cohort studies, and case studies.

Secondary (filtered or pre-appraised) literature: Analyzes and interprets groups of primary studies, including systematic reviews and meta-analyses,


Examples of Primary Study Types

Case Series/Case Report:  A report on the treatment of an individual patient. Because there is no control group for comparison, there is no statistical validity. A number of case reports is a case series.

Case-Control Study:  A study in which people who already have a condition are compared with those who do not. The researcher looks back over time to identify factors that might be associated with the condition. Often relies on medical data or patient recall and is less reliable than an RCT or cohort study because cause and effect is not necessarily established.

Cohort Study:  Follows patients who have a particular condition or receive a particular treatment over time, and compares them with another group who have not been affected by the condition or treatment being studied. Not as reliable as an RCT since the two groups might differ in ways other than the variable being studied. 

Randomized Controlled Study:  A carefully planned experiment that studies the effect of therapy on real patients. RCTs include methodologies (randomization and blinding) that reduce bias and that allow for comparison between an intervention group and a control group (no intervention). RCTs can provide sound evidence for cause and effect.

Systematic Review:  Brings together and distills the best evidence  from the primary literature to answer a clinical question. Generally, this will pool the results of several RCTs or meta-analyses on the same clinical problem. 

Meta-analysis:  A  quantitative statistical analysis of several separate but similar experiments or studies in order to test the pooled data for statistical significance.