Table 3.

Quality assessment of included studies based on the Newcastle Ottawa Scale (NOS)

	Selection (4)				Comparability (2)	Exposure (3)			Total score
	Is the case definition adequate (assessment of outcome)	Representativeness of the cases	Selection of controls (assessment of outcome)	Definition of controls (representativeness of the samples)	Adjusted for covariates	Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-response rate
Case-control studies
Czwornog et al. [54]	1	1	1	1	2	1	1	0	8
Choe et al. [53]	1	1	1	1	2	1	1	0	8
Lipka et al. [55]	1	1	0	1	2	1	1	0	7
Kim et al. [56]	1	1	1	1	2	1	1	0	8

	Selection (4)				Comparability (2)	Outcome (3)		Total score
	Representativeness of the samples	Sample size	Non-respondents	Ascertainment of the exposure	Adjusted for covariates	Assessment of the outcome (2)	Statistical test
Cross-sectional studies
Guilera et al. [43]	1	1	0	1	2	2	1	8
Kim et al. [44]	1	1	0	1	2	2	1	8
Kim et al. [45]	1	1	0	1	2	2	1	8
Nam et al. [24]	1	1	0	1	2	2	1	8
Stein et al. [46]	1	1	0	1	2	2	1	8
Kim et al. [47]	1	1	0	1	2	2	1	8
Kim et al. [48]	1	1	0	1	2	2	1	8
Yun et al. [49]	1	1	0	1	2	2	1	8
Lee et al. [50]	1	1	0	1	2	2	1	8
Wang et al. [51]	1	1	0	1	2	2	1	8
Murphy et al. [52]	0	1	0	1	2	2	1	7
Wong et al. [59]	1	1	0	1	2	2	1	8

	Selection (4)				Comparability (2)	Outcome (3)			Total score
	Representativeness of the exposed cohort	Selection of the non-ex-posed cohort	Ascertainment of exposure	Demonstration that outcome was not present at start of study	Adjusted for covariates	Assessment of outcome	Follow-up long enough for outcomes to occur	Adequacy of follow up of cohorts
Cohort studies
Sedjo et al [57]	1	1	1	1	2	1	1	0	8