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PICO Research Questions: Levels of Evidence

This guide provides tips for creating PICO research questions and searching the databases.

What Types of Evidence is Needed?

Evidence Pyramids rank research types based on the rigor of evidence provided in the study. High level evidence from the top of the pyramid is preferred but may not be available for all questions. The type of questions that you are asking determines which types of studies are most appropriate.

     Pyramid of Evidence

Watch this video to find out more about the types of literature in the Evidence Pyramid.

Filtered Information (Secondary Literature)  
Meta-analysis Systematic reviews that use quantitative methods to summarize results.
Systematic Review Rigorous methods of identifying, critically appraising, and synthesizing literature for a specific topic.
Critically Appraised Topic and Articles Evaluation/synthesis of multiple research studies or of an individual research article.
Unfiltered Information (Primary Literature)  
Randomized Controlled Trials (RCTs) Randomized group of patients in experimental and control groups. Followed p for the variables/outcomes of interest.
Cohort Study Two groups of patients that received exposure or not, followed forward for outcome of interest.
Case-Control Study Comparison of patients with outcome of interest and control patients without outcome to see if they had exposure of interest.
Background Information Handbooks, encyclopedias, textbooks, that provide good foundation or introduction to topic.


Different types of clinical questions can be best answered by different types of research studies. If you cannot find a study with the highest level of evidence, work your way down the Evidence Pyramid until you find a study on your topic. Here are study designs best suited to each type of clinical question:

Clinical Question Suggested Research Design
All Clinical Questions Systematic review, meta-analysis
Therapy Randomized controlled trial, meta-analysis
Etiology Randomized controlled trial, meta-analysis, cohort study
Diagnosis Randomized controlled trial
Prevention Randomized controlled trial, meta-analysis
Prognosis Cohort study
Meaning Qualitative study
Quality Improvement Randomized controlled trial
Cost Economic evaluation