Table 1

 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)
A1Systematic reviews that include at least some studies of level A2, in which the results of the separate studies are consistent
A2Randomised controlled clinical trial of good-quality, random, double-blind controlled trials, of sufficient size and consistency
BRandomised clinical trials of moderate quality or insufficient size or other comparative studies (not-randomised, comparative cohort study, patient–control study)
CNon-comparative studies
DOpinion of experts
Degree of evidence of the conclusions
1One systematic review (A1, or at least two independently conducted studies of degree of A1 or A2)
2At least two independently conducted studies of degree B
3One study of degree A2 or B or one or more studies of degree C
4Opinion of experts