3.2. Risk of bias
The quality of the studies is presented in Table 2 . Briefly, random sequencing generation was unclear in 3 studies (20, 30). Participants and outcome blinding and incomplete outcome data were high and unclear risk of bias in only two studies, respectively (20, 32). And then ranked as low risk of bias for 5 studies (7, 19, 29-31), medium risk of bias for 1 study (32), and high risk of bias for 1 study (20). Most of studies had a low risk of bias for selective reporting and other sources of bias except for one study (20).