Table 2.

Risk of bias assessment of the included studies according to the Newcastle-Ottawa Scale (NOS).

NOS items/Study ID	Hanes et al.	Xu et al.	Saugel et al.	Guo et al.	Scholte et al.	Shi et al.	Saied et al.
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	8	7	7	6	7	6	8

^*Representative studies meet this criteria.