Grading system used in the 2001 British Thoracic Society (BTS) guideline on diagnostic flexible bronchoscopy
| Level | Evidence |
|---|---|
| Ia | Evidence obtained from meta-analysis of RCTs |
| Ib | Evidence obtained from at least one RCT |
| IIa | Evidence obtained from at least one well designed controlled study without randomisation |
| IIb | Evidence obtained from at least one other type of well designed quasi-experimental study |
| III | Evidence obtained from well designed non-experimental descriptive studies such as comparative studies, correlation studies and case–controlled studies |
| IV | Evidence obtained from expert committee reports of opinions and/or clinical experiences of respected authorities |
| Grade | Type of recommendation |
| A (Ia, Ib) | Requires at least one RCT as part of a body of literature of overall good quality and consistency addressing the specific recommendation |
| B (IIa, IIb, III) | Requires availability of well conducted clinical studies but no RCTs on the topic of recommendation |
| C (IV) | Requires evidence from expert committee reports or opinions and/or clinical experience of respected authorities. Indicates absence of directly applicable studies of good quality |
RCT, randomised controlled trial.