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Levels of Evidence

Levels of Evidence

When searching for evidence-based information, one should select the highest level of evidence available. Clinicians must understand the types of evidence and their relative quality in the hierarchy of research evidence. The Levels of Evidence pyramid below represents the relative quality of types of evidence with the least clinically relevant at the bottom and the most clinically relevant at the top.  


                                          LEVELS OF EVIDENCE

Image adapted from: Murad MH, Asi N, Alsawas M, &Alhadab, F. (2016). New evidence pyramid. BMJ Evidence-Based Medicine, (English Ed.)21(4), 125–127.

Study Types

Systematic Reviews summarize the results of a systematic literature search on a specific clinical question to develop clinical recommendations. The studies are reviewed, assessed, and the results are summarized according to the predetermined criteria of the review question. They assess the methodology, sample size, and quality of the studies, using the highest quality data available to answer specific clinical questions and develop practice recommendations.

Meta-Analysis takes this process one-step further, reviewing a clinical question for which multiple systematic reviews exist and combining all the results using accepted statistical methodology  

Randomized, Controlled Clinical Trial - A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals who are similar at the beginning are randomly allocated to two or more groups (treatment and control) and the outcomes of the groups are compared after sufficient follow-up time.

A study that shows the efficacy of a diagnostic test is called a prospective, blind comparison to a gold standard study. This is 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.

Cohort Studies identify a large population who already has a specific exposure or treatment, follows them over time (prospective), and compares outcomes with another group that has not been affected by the exposure or treatment being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.

Case Control Studies are studies in which patients who already have a specific condition or outcome are compared with people who do not. Researchers look back in time (retrospective) to identify possible exposures. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other.

Case Series and Case Reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.

Background Information / Expert Opinion use varied evidence to present information that ranges from expert opinion to providing summaries of well-known information with established evidence. They are good resources to begin understanding a topic, learning definitions, and clinical parameters. However, when answering an EBP question, look for information with statistically significant data from resources higher-up in the pyramid.

Source: Duke University Medical Center Library: Evidence-Based Medicine Resources

Filtered Information vs Unfiltered Information

Filtered Information - Information that has been collected and aggregated by expert analysis and review. The quality of the studies has already been appraised and recommendations for practice may have already been made. Examples include systematic reviews and meta-analyses.. 

Unfiltered Information - Primary or original research studies. Original research studies that have not yet been synthesized or aggregated such as randomized controlled trials (RCTs), cohort studies and case-control studies, which are often published in peer-reviewed journals. These studies have not undergone additional analysis and review beyond that of the peer review process for each study.