Fig. 2

Assessment of the risk of bias. a Risk of bias graph and b Risk of bias summary; a represents the percentage of each bias level for five items, and b indicates the level of specific items in each study. Different colours (green, red, yellow) and symbols (“+”, “ - “, “?”) indicate “low risk of bias”, “high risk of bias” and “unclear risk of bias”. A study with three or more green “+” can be considered to have a low risk of bias