Table 2.

Assessment of quality and risk of bias by the Newcastle–Ottawa scale.

Cross sectional	Amiri et al. (17)	Karjalainen et al. (19)	Song et al. (16)	Veena et al. (11)	Ota et al. (18)
Selection (5 stars max)
1) Is the case definition adequate?	*	–	*	–	–
2) Sample	*	–	*	–	–
3) Non-respondents	–	–	*	*	–
4) Ascertainment of the exposure (risk factor)	**	*	**	*	*
Comparability (2 stars max)
1) The subjects in different outcome groups are comparable, based on the study design or analysis. Confounding factors are controlled	*	*	**	*	–
Outcome (3 stars max)
1) Ascertainment of outcome	**	**	**	**	**
2) Statistical test	*	*	*	-	*
Overall	Good Quality	Poor Quality	Good Quality	Fair Quality	Poor Quality

Domain evaluation score: one star: “*”; two stars: “**”; three stars: “***”. A “star system” has been developed to evaluate studies on three broad perspectives: the selection of the study groups; the comparability of the groups; and the ascertainment of either the exposure or outcome of interest for case-control, cross-sectional, or cohort studies, respectively.