Table 2.

The quality of the included study was evaluated according to the Newcastle-Ottawa scale (NOS).

Study ID/ NOS items	Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of controls	Compatibility	Ascertainment of exposure	Same method of ascertainment for cases and control	Non-response rate	Total score
Raffenberg et al.	*	*		*	*	*	*		6
Dai et al.	*	*		*	*	*	*		6
Nseir et al.	*	*		*	*	*		*	7
Ansari et al.	*	*	*	*	*	*	*		7
Xu et al.		*	*	*	*	*	*	*	7
Mutlu et al.	*	*		*	*	*	*		6
Saugel B et al.	*	*	*	*	*	*	*		7
Peng et al.	*	*		*	*	*	*	*	7
Guo et al.	*	*		*	*	*	*		6
Lv et al.	*	*		*	*		*		5
Hottal et al.	*	*		*	*	*	*	*	7
Cha et al.	*	*	*	*	*	*	*		7
Cabaret et al.	*	*		*	*	*	*		6
Sybille et al.	*	*			*	*	*	*	7
Liu et al.	*	*		*	*	*	*	*	7
Shi et al.	*		*	*	*	*	*	*	7
Ibn Saied et al.	*	*	*	*	*	*	*	*	8
Imoto et al.	*	*		*	*	*	*	*	7

^*Representative studies meet this criteria.