Levels of evidence

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-IV.


 * Ia - Evidence from Meta-analysis of Randomized Controlled Trials
 * Ib - Evidence from at least one Randomized Controlled Trial
 * IIa - Evidence from at least one well designed controlled trial which is not randomized
 * 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