Table 2. The quality assessment of all included studies based on the Newcastle–Ottawa Scale.

Categories	Items	Fang’s study	Cargnin’s study	Maher’s study	Formicola’s study
Selection	Adequacy of case definition	*	*	*	*
	Representativeness of the cases	-	-	*	*
	Selection of controls	*	*	*	*
	Definition of controls	*	*	*	*
Comparability	Comparability of cases/controls	*	*	*	*
Exposure	Ascertainment of exposure	*	*	*	*
	Same method of ascertainment for cases and controls	*	*	*	*
	Non-response rate	*	*	*	*