Levels of evidence and grading of recommendations7
| Level | Type of evidence |
| Ia | Meta-analysis of randomised controlled trials |
| Ib | At least one randomised controlled trial |
| IIa | At least one well designed controlled study without randomisation |
| IIb | At least one other type of well designed quasi-experimental study |
| III | From well designed non-experimental descriptive studies |
| IV | Expert committee reports of opinions and/or clinical experiences of respected authorities |
| Grade | Type of evidence |
| A (levels Ia,Ib) | Requires at least one randomised controlled trial |
| B (levels IIa, IIb, III) | Well conducted clinical studies but no randomised controlled trial |
| C (level IV) | Expert committee reports or opinion. Indicates absence of directly applicable studies of good quality |