Levels of evidence and grades of recommendation based on the Oxford Centre for Evidence Based Medicine
| Level | Individual study | Technique |
|---|---|---|
| For details see http://www.cebm.net/levels_of_evidence.asp#refs | ||
| 1a | Systematic review (SR) with homogeneity of level 1 diagnostic studies | Systematic review (SR) with homogeneity of randomised controlled trials (RCTs) |
| 1b | Validating cohort study with good reference standards | Individual RCT (with narrow confidence interval) |
| 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; for example, <80% followup) |
| 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” |
| Grades of recommendation | ||
| A | consistent level 1 studies | |
| B | consistent level 2 or 3 studies or extrapolations from level 1 studies | |
| C | level 4 studies or extrapolations from level 2 or 3 studies | |
| D | level 5 evidence or troublingly inconsistent or inconclusive studies of any level | |