Table 3.

Newcastle-Ottawa Scale for bias risk assessment of non-randomized studies.

Study	Selection				Comparability	Outcomes			Total
	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Definition that outcome of interest was not present at the start of study		Ascertainment of outcome	Was follow-up long enough for outcomes to occur	Adequacy of follow-up of cohorts
Park et al. (16)	1	1	1	1	2	1	0	0	7
Baek et al. (14)	1	1	1	1	1	1	0	0	6
Kuo et al. (18)	1	1	1	1	1	1	1	1	8
Yoo et al. (15)	1	1	1	1	1	1	0	1	7
Park et al. (17)	1	1	1	1	1	1	0	1	7