Table 3.

Quality of included studies

No.	Reference	Selection				Compatibility	Exposure
		Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of controls		Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-response rate
1.	Bottieau et al. [26]	*	*	*	*	**	*	*	*
2.	Chaparro et al. [23]	*	*	*	*	**	*	*	*
3.	Douglas et al.[21]	*		*	*	**	*	*	*
4.	Hwang et al. [24]	*	*	*	*	**	*	*	*
5.	Langford et al. [22]	*	*	*	*	**	*	*	*
6.	Wangdahl et al. [25]	*	*	*	*	**	*	*	*