Evidence level | Types of studies |
1 a | Systematic Review (with homogeneity) of Randomized Controlled Trial; |
1 b | Individual Randomized Controlled Trial (with narrow Confidence Interval); |
1 c | All or none case-series; |
2 a | Systematic Review (with homogeneity) of cohort studies; |
2 b | Individual cohort study (including low quality Randomized Controlled Trial; e.g., <80% follow-up); |
2 c | “Outcomes” Research; Ecological studies; |
3 a | Systematic Review (with homogeneity) of case-control studies; |
3 b | Individual Case-Control Study; |
4 | 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”. |