Fig. 2: Risk of Bias-summary.

Note: This figure displays the sum of all risk of bias for each included study. Light gray = potential bias is low; middle gray = potential bias in this domain is unclear (plausible bias that raises some doubt about the results of this study); dark gray = potential bias is high (plausible bias that seriously weakens confidence in the results of this study). Attrition bias is only applicable in prospective studies (n = 3).