Table 2. Quality assessment conducted according to the Newcastle–Ottawa Scale for all the included studies.

	Selection				Comparability	Exposure
Author	Adequate definition of case	Representativeness of the cases	Selection of controls	Definition of controls	Comparability of cases and controls	Exposure assessment	Same method of ascertainment for cases and controls	Non-response rate	Total score
Lu [10]	*	*		*		*	*	*	6
Amosco [11]	*	*		*	*	*	*	*	7
Salimi [12]	*	*		*	*	*	*	*	7
Silva [13]	*	*		*	*	*	*	*	7
Zhang Honghui	*	*		*		*	*		5
Procopciuc	*	*		*	*	*	*	*	7
Chedraui	*	*		*	*	*	*	*	7
Andraweere	*	*		*	*	*	*	*	7
Atis	*	*		*	*	*	*	*	7
Chen Baoli	*	*		*		*	*		5
He Yun	*	*		*		*	*	*	6
Garza-Veloz	*	*		*	**	*	*	*	8
Cunha	*	*		*	*	*	*	*	7
Liu Shifang	*	*		*	*	*	*		6
Huang Yuliang	*	*		*		*	*		5
Sandrim	*	*		*	**	*	*	*	8
Nagy	*	*		*	*	*	*	*	7
Shim	*	*		*	**	*	*	*	8
Kim	*	*		*	*	*	*	*	7
Banyasz	*	*		*	**	*	*	*	8
Papazoglou	*	*		*	**	*	*	*	8