| 1a | Systematic analysis (systematic review) of RCTs with homogenous results |
| 1b | Particular RCT with limited dispersion |
| 1c | Therapy, before its introduction all patients died |
| 2a | Systematic review of cohort studies with homogenous results |
| 2b | Particular cohort studies or RCT of lower quality |
| 2c | “Outcomes” research; ecological studies |
| 3a | Systematic review of case–control studies with homogenous results |
| 3b | Particular case–control study |
| 4 | Case studies and cohort studies or case–control studies of limited quality |
| 5 | Expert opinions |
RCT: randomised controlled trial.