Class of recommendation and level of evidence definitions used by major guideline bodies
| Classes of recommendations | Definition |
|---|---|
| Class I | Evidence and/or general agreement that a given treatment or procedure is beneficial, useful, effective. |
| Class II | Conflicting evidence and/or a divergence of opinion about the usefulness/efficacy of the given treatment or procedure. |
| Class IIa | Weight of evidence/opinion is in favour of usefulness/efficacy. |
| Class IIb | Usefulness/efficacy is less well established by evidence/opinion. |
| Class III | Evidence or general agreement that the given treatment or procedure is not useful/effective, and in some cases may be harmful. |
| Levels of evidence | Definition |
| Level of evidence A | Data derived from multiple randomised clinical trials or meta-analyses. |
| Level of evidence B | Data derived from a single randomised clinical trial or large non-randomised studies. |
| Level of evidence C | Consensus of opinion of the experts and/or small studies, retrospective studies, registries. |