Table 1 Levels of evidence for therapeutic interventions
From: Clinical review: Evidence-based perioperative medicine?
Level of evidence | Therapy/prevention, aetiology/harm | |
|---|---|---|
1 | A | Systematic review (with homogeneity*) of randomised controlled trials |
| Â | B | Individual randomised controlled trial (with narrow confidence interval) |
| Â | C | All or none |
2 | A | Systematic review (with homogeneity) of cohort studies |
| Â | B | Individual cohort study (including low-quality randomised controlled trial [e.g. < 80% follow-up]) |
| Â | C | 'Outcomes' research; ecological studies |
3 | A | Systematic review (with homogeneity*) of case–control studies |
|  | B | Individual case–control study |
4 |  | Case series (and poor-quality cohort and case–control studies) |
5 | Â | Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles' |