Table 2. Results of quality assessment using the Newcastle-Ottawa Scale for case-control studies.

Study	Selection					Comparability		Exposure			Scores
	Adequate definition of cases	Representativeness of the cases	Selection of controls	Definition of controls		Control for important factora		Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-response rate
Cao et al. (24)	☆	☆	☆	☆		☆☆		☆	☆	–	8
Zhu et al. (13)	☆	☆	☆	☆		☆☆		☆	☆	–	8
Cai et al. (29)	☆	☆	☆	☆		☆		☆	☆	–	7
Tong et al. (25)	☆	☆	☆	☆		☆		☆	☆	–	7
Bai et al. (26)	☆	☆	☆	☆		☆		☆	☆	☆	8
Song et al. (28)	☆	☆	☆	☆		☆		☆	☆	–	7
Li et al. (27)	☆	☆	☆	☆		☆☆		☆	☆	–	8
Xu et al. (14)	☆	☆	–	☆		☆		☆	☆	–	6

^a, a maximum of 2 stars can be allotted in this category, one for age, the other for other controlled factors. –, not available.