Strength of evidence assessment.
| 1 | Strong evidence | Consistent findings in multiple high-quality cohort and/or case–control studies. |
| 2 | Moderate evidence | Consistent findings in multiple cohort and/or case–control studies, of which only one is a high-quality study |
| 3 | Some evidence | Findings of one cohort or case–control study or consistent findings in multiple cross-sectional studies, of which at least one study is a high-quality study |
| 4 | Inconclusive evidence | Concerns all other cases—that is, consistent findings in multiple low-quality, cross-sectional studies or inconsistent findings in multiple studies. The evidence is considered to be inconclusive if only one cross-sectional study is available, regardless of the quality of this study |
Note: Findings are consistent when the results of at least 75% of the studies have similar outcomes.