Risk of bias assessment
Two reviewers assessed the risk of bias of the included studies according to the Cochrane Risk of Bias (ROB) tool (Cochrane Handbook Version 5.1.0)[21, 22]. The ROB tool included seven domains: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other biases. Each domain was judged as either yes, no, or unclear according to the information provided in the studies[23]. If all the domains were rated yes, the study was judged to be at low risk of bias, and if one or more domains were rated no, the study was judged to be at high risk of bias. Otherwise, the study was judged to have an unclear risk of bias. Disagreements in bias assessment were resolved by discussions among the two reviewers and, if necessary, through consultation with a third reviewer[24].