Categories of evidence16
| Ia Evidence from meta-analyses of randomised controlled trials |
| Ib Evidence from at least one randomised controlled trial |
| IIa Evidence from at least one controlled study without randomisation |
| IIb Evidence from at least one type of quasi-experimental study |
| III Evidence from descriptive studies, such as comparative, correlation, or case–control |
| IV Evidence from expert committee reports or opinions or clinical experience of respected authorities, or both |