Table 2.

Assessment of the quality of the studies based on the NOS

Study	Selection (out of 4)				Comparability (out of 2)	Outcomes (out of 3)			Total (out of 9)
	(1)	(2)	(3)	(4)		(5)	(6)	(7)
Zhang [21]	*	*	*	*	**	*			7
Liu [22]	*	*	*	*	**	*			7
Huang [23]	*	*	*	*	**	*			7
Zhou [24]	*	*	*	*	**	*	*	*	9
Son T [25]	*	*	*	*	**	*	*	*	9
Han [26]	*	*	*	*	**	*	*	*	9
Lee [27]	*	*	*	*	*	*	*	*	8
Suda [28]	*	*	*	*	*	*		*	7
Shen [29]	*	*	*	*	**	*			7
Hong [30]	*	*	*	*	**	*			7
Lan [31]	*	*	*	*	**	*			7
Zhang [32]	*	*	*	*	**	*	*	*	9
Li [33]	*	*	*	*	**	*	*		8
Obama [34]	*	*	*	*	**	*	*	*	9
Gao [35]	*	*	*	*	**	*	*	*	9
Sun [36]	*	*	*	*	**	*			7
Ye [37]	*	*	*	*	**	*			7
Kong [38]	*	*	*	*	**	*			7
Cui [39]	*	*	*	*	**	*			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) assessment of outcome, (6) was follow-up long enough for outcomes to occur, (7) adequacy of follow-up of cohorts