Table 2.

Quality assessment of the eligible observational cohort studies.

Studies included	Selection				Comparability		Outcome			Scores
	1	2	3	4	5	6	7	8	9
Hu et al., 2010 [25]	√	√	√	√	√	√		√		7
Gao et al., 2015 [26]	√	√	√	√	√	√		√	√	8
Cui et al., 2010 [22]	√	√	√	√	√			√		6
Chen et al., 2012 [27]	√	√	√		√			√		5
Lai et al., 2013 [28]	√	√	√		√	√		√		6
Zhang et al., 2014 [21]	√	√	√		√			√	√	6

For cohort studies, 1 indicates exposed cohort truly representative; 2 nonexposed cohort drawn from the same community; 3 ascertainment of exposure; 4 outcome of interest not present at start; 5 cohorts comparable based on the most important factors; 6 cohorts comparable on other factors; 7 quality of outcome assessment; 8 follow-up long enough for outcomes to occur; and 9 adequacy of follow-up of cohorts.