Table 2.

The quality assessment of included studies by the Newcastle-Ottawa Scale.

Study	Newcastle-Ottawa scale components								Quality score
	1	2	3	4	5	6	7	8
Wei et al. (16)	*	*	*	*		*	*	*	7
Domagalski et al. (17)		*	*	*		*	*	*	6
Limothai et al. (18)	*	*	*	*		*	*	*	7
Boglione et al. (19)	*	*	*	*		*		*	6
Domagalski et al. (20)	*	*	*	*		*	*	*	7
Wu et al. (14)	*	*	*	*		*	*	*	7
Zhang et al. (21)	*	*	*	*		*	*	*	7
Lampertico et al. (27)	*	*	*	*		*	*	*	7
Guo et al. (22)		*	*	*		*	*	*	6
Holmes et al. (15)	*	*	*	*		*	*	*	7
Wu et al. (23)	*	*	*	*		*	*	*	7
de Niet et al. (24)	*	*	*	*		*	*	*	7
Tseng et al. (25)	*	*	*	*		*	*	*	7

1, Representativeness of the exposed cohort; 2, Selection of the non-exposed cohort; 3, Ascertainment of exposure; 4, Demonstration that outcome of interest was not present at start of study; 5, Comparability of cohorts on the basis of the design or analysis; 6, Assessment of outcome; 7, Was follow-up long enough for outcomes to occur; 8, Adequacy of follow up of cohorts.

^*p < 0.05.