Categories of levels of evidence
| Category | Level of evidence |
|---|---|
| Ia | Meta-analysis of randomised controlled trials |
| Ib | At least one randomised controlled trial |
| IIa | At least one controlled trial without randomisation |
| IIb | At least one type of quasi-experimental study |
| III | Descriptive studies, such as comparative studies, correlation studies or case–control studies |
| IV | Expert committee reports or opinions and/or clinical experience of respected authorities |