Classification of the substantiation in level and degree of evidence in the conclusions of the studies
| 1, strong evidence; 2, moderate evidence; 3, poor or conflicting evidence; 4, no evidence. | |
| Source: Dutch Institute for Healthcare Improvement.23 | |
| For articles concerning: intervention (prevention or therapy) | |
| A1 | Systematic reviews that include at least some studies of level A2, in which the results of the separate studies are consistent |
| A2 | Randomised controlled clinical trial of good-quality, random, double-blind controlled trials, of sufficient size and consistency |
| B | Randomised clinical trials of moderate quality or insufficient size or other comparative studies (not-randomised, comparative cohort study, patient–control study) |
| C | Non-comparative studies |
| D | Opinion of experts |
| Degree of evidence of the conclusions | |
| 1 | One systematic review (A1, or at least two independently conducted studies of degree of A1 or A2) |
| 2 | At least two independently conducted studies of degree B |
| 3 | One study of degree A2 or B or one or more studies of degree C |
| 4 | Opinion of experts |