Evidence hierarchy and traditional strength of recommendation
| Category of evidence | Strength of recommendation | |
|---|---|---|
| Ia Meta-analysis of RCTs | A Category I evidence | |
| Ib RCT | ||
| 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 |