Levels of Evidence

Ia - Evidence obtained from meta-analysis or systematic review 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.