Levels of Evidence and Grades of Recommendation Table
| Study criteria | Level of evidence | Criteria for grading of recommendation | Grade of recommendation |
| Systematic review (with homogeneity) of randomised, controlled trials | 1a | Consistent level 1 studies | A |
| Individual, randomised, controlled trials with statistically significant results | 1b | ||
| All or none, i.e. prior to availability of new therapy, all died, now with therapy some survive; or, prior to therapy some died, now with therapy none die | 1c | ||
| Systematic review (with homogeneity) of cohort studies | 2a | Consistent level 2 or 3 studies (or extrapolations* from level 1 studies) | B |
| Individual cohort study (including low quality randomised controlled trial, e.g. <80% follow up) | 2b | ||
| Outcomes research | 2c | ||
| Systematic review (with homogeneity) of case-controlled studies | 3a | ||
| Individual case-controlled study | 3b | ||
| Case-series, and poor quality cohort and case-controlled studies | 4 | Level 4 studies (or extrapolations* from level 2 or 3 studies) | C |
| Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles | 5 | Level 5 evidence (or troublingly inconsistent or inconclusive studies of any level) | D |
| *Extrapolations are where data are used in a situation that has potentially clinically important differences to the original study situation. In the case of PROSPECT, extrapolation largely refers to transferable evidence. | |||