Levels of evidence and grading of recommendations based on the US Agency for Health Care Policy and Research (AHCPR)10
| Level | Type of evidence |
| Ia | Evidence obtained from meta-analysis of randomised controlled trials |
| Ib | Evidence obtained from at least one randomised controlled trial |
| IIa | Evidence obtained from at least one well designed controlled study without randomisation |
| IIb | Evidence obtained from at least one other type of well designed quasi-experimental study |
| III | Evidence obtained from well designed non-experimental descriptive studies such as comparative studies, correlation studies, and case controlled studies |
| IV | Evidence obtained from expert committee reports of opinions and/or clinical experiences of respected authorities |
| Grade | Type of recommendations |
| A (levels Ia, Ib) | Requires at least one randomised controlled trial as part of a body of literature of overall good quality and consistency addressing the specific recommendation |
| B (levels IIa, IIb, III) | Requires availability of well conducted clinical studies but no randomised clinical trials on the topic of recommendation |
| C (level IV) | Requires evidence from expert committee reports or opinions and/or clinical experience of respected authorities. Indicates absence of directly applicable studies of good quality |