Brief description of the generic levels of evidence and guideline statement grades used
| Evidence level | Definition | Guideline statement grade |
| Ia | A good recent systematic review of studies designed to answer the question of interest | A+ |
| Ib | One or more rigorous studies designed to answer the question, but not formally combined | A− |
| II | One or more prospective clinical studies which illuminate, but do not rigorously answer, the question | B+ |
| III | One or more retrospective clinical studies which illuminate, but do not rigorously answer, the question | B− |
| IVa | Formal combination of expert views | C |
| IVb | Other information | D |