Classification and grading of clinical evidence
| Table adapted from Shekelle et al.7 | |
| Category of evidence | |
| Ia | Evidence for meta-analysis from 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 other type of quasi-experimental study |
| III | Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies |
| IV | Evidence from expert committee reports or opinions or clinical experience of respected authorities, or both |
| Grade: Strength of recommendation | |
| A | Directly based on category I evidence |
| B | Directly based on category II evidence or extrapolated from category I evidence |
| C | Directly based on category III evidence or extrapolated from category I or II evidence |
| D | Directly based on category IV evidence or extrapolated from category I, II, or III evidence |