Hierarchy of Evidence
A procedure for labelling the strength of the evidence in support of the use of drugs and other medical products and procedures. It is widely used in systematic reviews and is a pillar of evidence-based medicine. A ranking might be as follows:
Ia Evidence from systematic reviews or meta-analysis of randomized controlled trials.
Ib Evidence from at least one randomized controlled trial.
IIa Evidence from at least one controlled study without randomization.
IIb Evidence from at least one other type of quasi-experimental study.
III Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies and case-control studies.
IV Evidence from expert committee reports or opinions and/or clinical experience of respected authorities.
There is much controversy over such classifications, much of which arises because the strength attributed will depend, amongst other things, on the purpose of the review. For example, if the purpose were to minimize bias (sometimes called internal validity), a hierarchy might serve that would be less useful were the purpose to assess the validity of the research as a predictor of what might happen in real-world practice (sometimes referred to as external validity).