Levels of Evidence

Ia  Evidence obtained from meta-analysis 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.