Table 1.

Quality assessment of included studies

Studies	Selection				Comparability	Outcome			Total scores
	Representativeness of the exposed group	Selection of the unexposed group	Ascertainment of exposure	Outcome of interest not present at start of study	Control for important factor or additional factor	Outcome assessment	Follow-up long enough for outcome to occur	Adequacy of follow-up of cohorts
Wang Qingtu, 2001 [45]	1	1	1	1	2	1	–	–	7
Wang Jianshe, 2002 [16]	1	1	1	1	2	1	1	1	9
Wang Lan, 2004 [44]	1	1	1	1	–	1	1	1	7
Wang Yuan, 2005 [43]	1	1	1	1	2	1	1	1	9
Gu Jie, 2006 [42]	1	1	1	1	2	1	1	1	9
Qian Yanhua, 2006 [41]	1	1	1	1	1	1	1	1	8
Fan Yi, 2007 [40]	1	1	1	1	2	1	1	1	9
Chen Jing1, 2007 [38]	1	1	1	1	1	1	1	1	8
Chen Jing2, 2007 [38]	1	1	1	1	1	1	1	1	8
Li Jijun1, 2007 [39]	1	1	1	1	1	1	1	1	8
Li Jijun2, 2007 [39]	1	1	1	1	1	1	1	1	8
Chen Hong, 2008 [37]	1	1	1	1	1	1	1	1	8
Liu Xia, 2008 [36]	1	1	1	1	–	1	1	1	7
Liu Honge, 2009 [35]	1	1	1	1	1	1	1	1	8
Zhang Qingying, 2009 [34]	1	1	1	1	2	1	1	1	9
Zhu Yunxia, 2010 [32]	1	1	1	1	2	1	1	1	9
Yang Peifang, 2010 [33]	1	1	1	1	2	1	1	1	9
Yang Xiaomei, 2011 [30]	1	1	1	1	2	1	1	1	9
Zhang Weili, 2011 [31]	1	1	1	1	1	1	1	1	8
Huang Liu, 2012 [29]	1	1	1	1	2	1	1	1	9
Guo Zhen, 2013 [15]	1	1	1	1	2	1	1	1	9
Wang Ling, 2013 [28]	1	1	1	1	2	1	1	1	9
Hu Yali, 2013 [14]	1	1	1	1	2	1	1	1	9
Yin Yuzhu, 2013 [12]	1	1	1	1	2	1	1	1	9
Pan Calvin Q., 2013 [13]	1	1	1	1	1	1	1	1	8
Zhang Lei, 2014 [11]	1	1	1	1	2	1	1	1	9
Wang Lina, 2014 [26]	1	1	1	1	2	1	1	1	9
Li Weiru, 2014 [25]	1	1	1	1	2	1	1		9
Huang Dan, 2014 [27]	1	1	1	1	2	1	1	1	9
Liu Cuiping, 2015 [10]	1	1	1	1	2	1	1	1	9