Evidence levels (EL) and recommendation grades (RG) (adapted from the Oxford Centre for Evidence Based Medicine, http://www.cebm.net/index.aspx?o=1025 (accessed Oct 2010))
| EL | Individual study | Technique |
| 1a | Systematic review (SR) with homogeneity of level 1 diagnostic studies | Systematic review (SR) with homogeneity of randomised controlled trials (RCTs) |
| 1b | Validating cohort studies with good reference standards | Individual RCT (with narrow CI) |
| 1c | Specificity is so high that a positive result rules in the diagnosis (SpPIn) or sensitivity is so high that a negative result rules out the diagnosis (SnNout) | All or none |
| 2a | SR with homogeneity of >level 2 diagnostic studies | SR (with homogeneity) of cohort studies |
| 2b | Exploratory cohort study with good reference standards | Individual cohort study (including low quality RCT; eg, <80% follow-up) |
| 2c | 'Outcomes' research; ecological studies | |
| 3a | SR with homogeneity of 3b and better studies | SR with homogeneity of case-control studies |
| 3b | Non-consecutive study; or without consistently applied reference standards | Individual case–control study |
| 4 | Case–control study, poor or non-independent reference standard | Case series (and poor quality cohort and case–control studies) |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, 'bench research' or 'first principles' | Expert opinion without explicit critical appraisal, or based on physiology, 'bench research' or 'first principles' |
| RG | GRADES OF EVIDENCE | |
| A | Consistent level 1 studies | |
| B | Consistent level 2 or 3 studies or extrapolation from level 1 studies | |
| C | Level 4 studies or extrapolation from level 2 or 3 studies | |
| D | Level 5 evidence or troublingly inconsistent or inconclusive studies of any level |