Levels of Evidence

Ia

Evidence obtained from meta-analysis or systematic reviews of randomized controlled trials.

Ib

Evidence obtained from at least one randomized controlled trial.

IIa

Evidence obtained from at least one well-designed controlled study without randomization.

IIb

Evidence obtained from at least one other type of well-designed quasi-experimental study, without randomization.

III

Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies and case studies.

IV

Evidence obtained from expert committee reports or opinions and/or clinical experiences of respected authorities.