Table 4. The results of studies quality score based on NOS checklist.

Questions/articles	Wei-hua Zhao[15]	Shuai Xiao [16]	Qiang Chen [17]	Zhumei Shi [18]	Liwen An [19]	Qiaoli Wu [20]	Xiangrong Liu [21]	Alenka Matjašič [22]	Adam Fowler [23]	Wanghao Chen [24]	Debora Olioso [25]	L.-Q. Wang [26]	Alessandra Santangelo [27]
Case definition	*			*		*	*	*	*	*	*	*	*
Representativeness of the cases				*		*		*	*	*	*	*	*
Selection of Controls
Definition of Controls				*	*		*	*	*	*		*	*
Comparability of cases and controls	*	*	*	*	*	*	*	*	*	*	*	*	*
Ascertainment of exposure	*	*	*	*	*	*	*	*	*	*	*	*	*
Same method of ascertainment for cases and controls	*	*	*	*	*		*	*	*	*	*	*	*
Non-Response rate	*	*	*	*	*	*	*	*	*	*	*	*	*
Final score	5	4	4	7	5	5	6	7	7	7	6	7	7