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Systematic Reviews: Levels of Evidence

A guide to finding resources for a systematic review in the health and medical sciences

Levels of Evidence

"Levels of Evidence" tables have been developed which outline and grade the best evidence. However, the review question will determine the choice of study design.

Secondary sources provide analysis, synthesis, interpretation and evaluation of primary works. Secondary sources are not evidence, but rather provide a commentary on and discussion of evidence. e.g. systematic review

Primary sources contain the original data and analysis from research studies. No outside evaluation or interpretation is provided. An example of a primary literature source is a peer-reviewed research article. Other primary sources include preprints, theses, reports and conference proceedings.

Levels of evidence for primary sources fall into the following broad categories of study designs (listed from highest to lowest):

  • Experimental: RTC's (Randomised Control Trials)
  • Quasi-experimental studies (Non-randomised control studies, Before-and-after study, Interrupted time series)
  • Observational studies (Cohort study, Case-control study, Case series) 

Based on information from Centre for Reviews and Dissemination. (2009). Systematic reviews: CRD's guidance for undertaking reviews in health care. Retrieved from

Hierarchy of Evidence Pyramid

"Levels of Evidence" are often represented in as a pyramid, with the highest level of evidence at the top:




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EBM and Study Design

An introduction to EMB and study design by Connie Schardt (Medical Center Library, Duke University).

Types of Study Design

The following definitions are from the Glossary on the Centre for Evidence Based Medicine (CEBM), University of Oxford, website: 

  • Systematic Review
    The application of strategies that limit bias in the assembly, critical appraisal, and synthesis of all relevant studies on a specific topic and research question. 
  • Critically appraised topic
    A short summary of an article from the literature, created to answer a specific clinical question.
  • Randomized control clinical trial (RCT)
    A 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
    A systematic review which uses quantitative methods to summarise the results

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