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 |
A fuller description is given in Section 1 and Appendices 1–4.