Evidence hierarchy and traditional strength of recommendation
| Category of evidence | Strength of recommendation |
|---|---|
| Ia Meta-analysis of randomised controlled trials | A Category I evidence |
| Ib Randomised controlled trial | |
| IIa Controlled study without randomisation | B Category II evidence or extrapolated from category I evidence |
| IIb Quasi-experimental study | |
| III Non-experimental descriptive studies, such as comparative, correlation, and case-control studies | C Category III evidence or extrapolated from category I or II evidence |
| IV Expert committee reports or opinion or clinical experience of respected authorities, or both | D Category IV evidence or extrapolated from category II or III evidence |