Table 3.

Quality scores of case-control studies using Newcastle-Ottawa Scale (maximum score of 9).

Reference	Selection				Comparability	Outcome
	Adequate definition of cases	Representativeness of cases	Selection of controls	Definition of controls	Comparability on the basis of the design or analysis	Assessment of exposure	Same method of ascertainment for cases and controls	Nonresponse rate (<20%)	Overall
Oertel et al., 2002 [8]	1	1	0	1	1	1	1	1	7
Sun et al., 2011 [12]	1	1	0	1	1	1	1	1	7
Tong et al., 2012 [10]	1	1	0	1	2	1	1	1	8
Yuan et al., 2012 [11]	1	1	0	1	2	1	1	1	8
Tian et al., 2010 [9]	1	1	0	1	2	1	1	1	8
White et al., 2009 [13]	1	1	0	1	2	1	1	1	8

0 = “no,” “unable to determine,” or “not available.”