| A |
Ia |
Meta-analysis of randomized controlled trials (RCTs) |
| Ib |
Single RCT (with narrow confidence interval) |
| Ic |
All-or-none principle |
| B |
IIa |
Meta-analysis of well-designed cohort studies |
| IIb |
Single well-designed cohort study or RCT of lesser quality |
| IIc |
Outcome research |
| IIIa |
Meta-analysis of case-control studies |
| IIIb |
Single case-control study |
| C |
IV |
Case series or cohort/case-control studies of lesser quality |
| D |
V |
Expert opinion without explicit evaluation of scientific evidence or based on physiological models or laboratory research |
