A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. There is broad agreement on the relative strength of large-scale, epidemiological studies. More than 80 different hierarchies have been proposed for assessing medical evidence. The design of the study (such as a case report for an individual patient or a blinded randomized controlled trial) and the endpoints measured (such as survival or quality of life) affect the strength of the evidence. In clinical research, the best evidence for treatment efficacy is mainly from meta-analyses of randomized controlled trials (RCTs). Typically, systematic reviews of completed, high-quality randomized controlled trials – such as those published by the Cochrane Collaboration – rank as the highest quality of evidence above observational studies, while expert opinion and anecdotal experience are at the bottom level of evidence quality. Evidence hierarchies are often applied in evidence-based practices and are integral to evidence-based medicine (EBM).

 

Note:   The above text is excerpted from the Wikipedia article Hierarchy of evidence, which has been released under the GNU Free Documentation License.