A<unk>  (or levels of<unk>  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<unk> es have been proposed for assessing medical<unk> . 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<unk> . In clinical research, the best<unk>  for treatment efficacy is mainly from meta-analyses of randomized controlled trials (RCTs). Systematic reviews of completed, high-quality randomized controlled trials – such as those published by the Cochrane Collaboration rank the same as systematic review of completed high-quality observational studies in regard to the study of side effects.<unk> <unk> es are often applied in<unk> -based practices and are integral to<unk> -based medicine (EBM).
Hierarchy of evidence
Theory of indispensable attributes
Causal graph
Adaptive design 
Scientific consensus
Causal model
Normalization model
Bead theory