| Category | Evidence |
| 1A | From meta-analysis of randomised controlled trials |
| 1B | From at least one randomised controlled trial |
| 2A | From at least one controlled study without randomisation |
| 2B | From at least one type of quasi-experimental study |
| 3 | From descriptive studies, such as comparative studies, correlation studies, or case–control studies |
| 4 | From expert committee reports or opinions and/or clinical experience of respected authorities |
Trial methodology and other uncontrolled results from any of the studies (including randomised controlled trials) were awarded a lower level of evidence.