Levels of evidence and recommendations2
| Hierarchy of evidence | Grading of recommendations | ||
|---|---|---|---|
| Level | Type of evidence | Level | Type of evidence |
| Ia | Evidence from systematic reviews or meta-analysis of randomised controlled trials | A | Based on hierarchy I evidence |
| Ib | Evidence from at least one randomised controlled trial | ||
| IIa | Evidence from at least one controlled study without randomisation | B | Based on hierarchy II evidence or extrapolated from hierarchy I evidence |
| 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 | C | Based on hierarchy III evidence or extrapolated from hierarchy I or II evidence |
| IV | Evidence from expert committee reports or opinions and/or clinical experience of respected authorities | D | Directly based on hierarchy IV evidence or extrapolated from hierarchy I, II or III evidence |
| NICE | Evidence from NICE guidelines or Health Technology Appraisal programme | NICE | Evidence from NICE guidelines or Health Technology Appraisal programme |
| HSC | Evidence from health service circulars | HSC | Evidence from health service circulars |