Table 2.

Study quality assessment (Newcastle–Ottawa scale).

Study	Selection				Comparability	Outcome			Total no. of stars
	Exposed cohort	Nonexposed cohort	Ascertainment of exposure	Outcome of interest		Assessment of outcome	Length of follow-up	Adequacy of follow-up
Cohort studies
Okada et al. [19]	∗	∗	∗	∗	∗∗	∗	∗	∗	9
Bradshaw et al. [21]	∗	∗	∗	∗	∗∗	∗	∗	∗	9
Kuiper et al. [22]	∗	∗	∗	∗	∗∗	∗	∗	—	8
Hardee et al. [23]	∗	∗	—	∗	∗∗	∗	∗	∗	8
Bao et al. [24]	∗	∗	∗	∗	∗∗	—	∗	∗	8
Gunnell et al. [25]	∗	∗	—	∗	∗∗	∗	∗	∗	8
Baade et al. [26]	∗	∗	∗	∗	—	∗	∗	∗	7
Irwin et al. [27]	—	∗	∗	∗	∗∗	∗	∗	∗	8
Case-control studies
Study	Selection				Comparability	Exposure			Total no. of stars
	Definition of cases	Representativeness of cases	Selection of controls	Definition of controls		Assessment of outcome	Method of ascertainment	Nonresponse rate
Abbott et al. [20]	∗	∗	∗	∗	∗∗	∗	∗	—	8