Table 2.

Quality assessment based on the NOS for observational studies

Author	Selection (out of 4)				Comparability (out of 2)	Outcomes (out of 3)			Total (out of 9)
	Representativeness of exposed cohort	Selection of non-exposed cohort	Ascertainment of exposure	Outcome not present at the start of the study		Assessment of outcomes	Length of follow-up	Adequacy of follow-up
Song	*	*	*	*	**	*			7
Ikeda	*	*	*	*	**	*			7
Kinoshita	*	*	*	*	**	*		*	8
Kim MG	*	*	*	*	*	*			6
Lee	*	*	*	*	**	*			7
Kim DG	*	*	*	*	*	*		*	7
Kim HG	*	*	*	*	*	*			6
Choi	*	*	*	*	**	*			7
Chen	*	*	*	*	**	*	*	*	9
Zhang	*	*	*	*	**	*			7
Han	*	*	*	*	*	*			6
Kanaji	*	*	*	*	**	*			7