Classification scheme for category of evidence and strength of recommendation
| Category of evidence | |
| Ia | Evidence from the meta-analysis 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 other type of quasi-experimental study |
| III | Evidence from non-experimental descriptive studies, such as comparative, correlation and case–control studies |
| IV | Evidence from expert committee reports, opinions or clinical experience of respected authorities, or both |
| Strength of recommendation | |
| A | Directly based on category I evidence |
| B | Directly based on category II evidence or extrapolated recommendation from category I evidence |
| C | Directly based on category III evidence or extrapolated recommendation from category I or II evidence |
| D | Directly based on category IV evidence or extrapolated recommendation from category I, II or III evidence |
This classification scheme was adopted from ‘Clinical guidelines: Developing guidelines’ by Shekelle et al (1999).