Levels of evidence is a ranking system used in evidence-based practices to describe the strength of the results measured in a clinical trial or research study. The design of the study (such as a case report for an individual patient or a double-blinded randomized controlled trial) and the endpoints measured (such as survival or quality of life) affect the strength of the evidence. Levels of evidence range from I-V.
Ia - Evidence from Meta-anlysis of Randomised Controlled Trials Ib - Evidence from at least one Randomised Controlled Trials IIa - Evidence from at least one well designed controlled trial which is not randomised IIb- Evidence from at least one well designed experimental trial III - Evidence from case, correlation, and comparative studies. IV - Evidence from a panel of experts
This article incorporates public domain material from the U.S. National Cancer Institute document "Dictionary of Cancer Terms".