Table 2 Evidence categories for observational studies - based on Ariëns et al. [18]

1

Strong evidence: Consistent findings in multiple high-score cohort and/or case- control studies.

2

Moderate evidence: Consistent findings in multiple cohort or case–control studies, of which only one study was of high quality.

3

Some evidence: Findings of one cohort or case–control study, or consistent findings in multiple cross-sectional studies, of which a least one study was of high quality.

4

Inconclusive evidence: All other cases (i.e. consistent findings in multiple low-quality cross-sectional studies, or inconsistent findings in multiple studies).

Moreover, inconclusive evidence was defined as findings of only one cross-sectional study, irrespective of the quality of the study.