Table 2. Quality assessment for all of the included studies.

First author	Publishing year	Selection	Comparability	Exposure	Total
Ni	2015	★★★	NA	★★	5
Li	2015	★★★	★	★★	6
Li	2014	★★★	★★	★★	7
Pei	2013	★★★	★	★★	6
Liu	2012	★★★	★★	★★	7
Qiu	2011	★★	★	★★	5
Sun	2007	★★★	★	★★	6
Yang	2010	★★★	NA	★★	5
A	2007	★★★	★	★★	6