The Evidence Pyramid

Acquiring the highest quality and most relevant studies for a research question is the second step of evidence-based practice. Use the evidence pyramid to help you to determine the best study types to answer your PICO clinical question.

Levels of Evidence

Studies can be categorized based on the strength of the evidence and methodology. The levels of evidence can be visualized as a pyramid, with the strongest and most reliable evidence at the top and weaker evidence at the bottom. The amount of available evidence increases as you move down the pyramid, but decreases in quality.

Secondary Sources

The top of the pyramid represents the strongest evidence. These studies review, evaluate and synthesize evidence rather than report new data. Secondary sources provide an overview of current research on a topic and inform evidence-based clinical decision making.

• Practice guidelines

  Recommendations about best practice. They explain how to diagnose and manage a medical condition based on the evidence presented in higher-level subject research such as systematic reviews, meta-analyses or comprehensive original research studies.
• Meta-analyses

  A meta-analysis is a report combining multiple studies on the same question in order to validate and strengthen conclusions using statistical analysis.
• Systematic reviews

  A systematic review seeks to answer a specific research question by identifying, appraising and synthesizing previously published research.

Primary Sources

Further down the pyramid are reports on original research that present new data or findings. You will need to read, interpret and evaluate these studies for application to practice.

• Randomized controlled trials (RCT)

  A clinical trial where participants are randomly assigned to either a treatment group or a control group. The test-treatment group receives the treatment being studied, while the control group receives either an alternative treatment, no treatment, or a placebo. Effects of the treatments are monitored to determine the efficacy of the treatment while reducing bias in both researchers and participants.
• Cohort studies

  A cohort study is an observational study of groups selected by their exposure to factors hypothesized to influence occurrence of a particular disease or other outcome.
• Case control studies

  Observational studies that comparisons compare persons with a disease or outcome of interest, and a control group without these factors. Researchers look to identify and compare past risk factors or exposures between the two groups.
• Case series and case reports

  Detailed reports of the symptoms, signs, diagnosis, treatment, and follow-up of an individual patient, that usually describes an unusual or novel occurrence. Case reports are one of the cornerstones of medical progress and provide new ideas in medicine.
• Background information & expert opinion

  Information that can often be found in point-of-care resources, medical textbooks, handbooks, encyclopedias, and reference materials. Consult when you need general information about a condition, medication, or diagnostic test.
  
  Expert opinion is a belief or interpretation by specialists with experience in a specific area.

Some study designs are better suited to answer different types of clinical questions.  You might not always find the highest level of evidence to answer your question. When this happens, work your way down to the next highest level of evidence.

Questions & Help

If you have questions on this, or another topic, contact a librarian for help!