Table 1. Levels of evidence for studies of treatment efficacy, ranked according to quality and credibility from highest/most credible (Ia) to lowest/least credible (IV) (adapted from the Scottish Intercollegiate Guideline Network, www.sign.ac.uk).
| Level | Description |
|---|---|
| Ia | Well-designed meta-analysis of >1 randomized controlled trial |
| Ib | Well-designed randomized controlled study |
| IIa | Well-designed controlled study without randomization |
| IIb | Well-designed quasi-experimental study |
| III | Well-designed nonexperimental studies, i.e., correlational and case studies |
| IV | Expert committee report, consensus conference, clinical experience of respected authorities |