Fig. 2

Risk of bias assessments by study. Legend: We assessed risk of bias as high (red circles with “−” ), some concern (yellow circles with “?”) or low (green circles with “+”) or according to the definitions outlined in the Cochrane Handbook [18]. A study was considered to have high overall risk of bias if they had one or more item with a high risk of bias. A study was considered to have some concern overall if they had one or more items with some concern, but no items with high risk of bias. A study was considered to have low overall risk of bias if all items had a low risk of bias