Table 3. Quality assessment of the included studies.

Study	Random sequence generation (selection bias)	Allocation concealment (selection bias)	Blinding of participants and personnel (performance bias)	Blinding of outcome assessment (detection bias)	Incomplete outcome data (attrition bias)	Selective reporting (reporting bias)	Other bias
Zhou et al.	+	+	−	?	+	+	+
Varpula et al.	?	+	−	?	+	?	?
Li et al.	+	−	−	?	+	+	+
Putensen et al.	?	?	−	?	+	?	−
Zhou et al.	+	?	−	?	+	?	+
Hirshberg et al.	+	+	−	?	+	+	+
Li et al.	+	?	−	?	+	+	?

+, low risk of bias; −, high risk of bias; ?, unclear risk.