Table 2.

The Risk of Bias and Quality of All Included Studies.

Study	Selection				Comparability	Outcome			NOS
	Representativeness of the exposed cohort	Selection of the nonexposed cohort	Ascertainmentof exposure	Demonstration that outcome of interest was not present at start of study	Comparability of cohort based on the design or analysis	Assessmentof outcome	Was follow-up long enough for outcome to occur	Adequate offollow up ofcohort
Du et al. (24)	1	1	1	0	0	1	1	1	6
Peng et al. (25)	1	1	1	0	2	0	1	1	7
Guo et al. (26)	1	1	1	0	2	0	1	1	7
Jin et al. (27)	1	1	1	1	0	1	1	1	7
Zhang et al. (28)	1	1	1	0	2	0	1	1	7
Liu et al. (29)	1	1	1	0	2	0	1	1	7
Study	Random sequence generation		Allocation concealment		Blinding of participants and personnel	Incomplete outcome data		Selective reporting	Other bias
Li et al. (30)	+		+		?	?		+	+