eTable 2. Adapted evidence classification according to SIGN (6).
| Type of study | Categories | Risk of systematic errors | Descriptive quality |
| Systematic review with randomized controlled trials | 1+ | Low | Well-conducted |
| 1− | High | - | |
| 1++ | Very low | High-quality | |
| Systematic review with cohort or case–control studies | 2++ | Very low | High-quality |
| 2+ | Low | Well-conducted | |
| 2− | High | - | |
| Randomized controlled trial | 1++ | Very low | High-quality |
| 1+ | Low | Well-conducted | |
| 1− | High | - | |
| Cohort or case–control study | 2+ | Low | Well-conducted |
| 2− | High | - |
-, no verbal description given