Evidence pyramid

The evidence hierarchy demonstrates the different levels of research literature. As you move up the pyramid, the study design is more rigorous and allows for less bias or systematic error. The amount of available literature decreases as you move up the pyramid, but increases in its relevance to the clinical setting.

Evidence pyramid

  • Filtered information are summaries or appraisals of the literature, and often make recommendations for practice.
  • Unfiltered information are original research studies. You will need to read, interpret, and evaluate their application to practice.


Types of study designs you will encounter include:

Systematic review Article in which the authors have systematically searched for, appraised, and summarised all of the medical literature for a specific topic.
Critically appraised topic Short summary of an article from the literature, created to answer a specific clinical question.
Randomised controlled clinical trial Group of patients is randomised into an experimental group and a control group. These groups are followed up for the variables/outcomes of interest.
Cohort study Involves the identification of two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
Case-control study Involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking to see if they had the exposure of interest.
Meta-analysis Quantitative summary of results which may be included in a systematic review.

(definitions from the Centre for Evidence Based Medicine): 

Find the best evidence for your question

The best study design to answer your question will also depend on the type of question you have. Always look for the highest level of evidence first. If the highest level of study to answer your question is not available, consider moving down the pyramid to other types of studies.

Type of question Definition Best type of study/research design
Therapy* Determining the effect of interventions on patients RCT
Diagnosis Establishing the power of a test to differentiate between those with and without a target condition or disease Prospective, blind comparison to a gold standard**
Harm / Etiology Ascertaining the effects of potentially harmful agents Cohort or case control
Prognosis Estimating a patients likely course over time due to factors other than interventions Cohort study
Prevention* Reducing the chance of disease by identifying and modifying risk factors, or diagnosing early through screening RCT
Clinical examination Gathering and interpreting findings from the history and physical examination Prospective, blind comparison to a gold standard**
Cost analysis Comparing the cost and consequences of different treatments and tests Economic analysis
Point of view Patient experience or concerns Qualitative study

*  Questions of therapy and prevention that can be best answered by a RCT can also be answered by a meta-analysis or systematic review.
** A controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests - the test under investigation and the “gold standard” test - to all of the patients in the study group.