Table 1. Overview of bias assessment results using the modified Newcastle-Ottawa scale for included studies: a cohort study.

	Selection				Comparability		Outcome			Total quality score	Quality
Study	Representativeness of exposed cohort	Selection of nonexposed cohort	Ascertainment of exposure	Demonstration that outcome of interest was not present at the start of the study	Adjust for the most important risk factors	Adjust for other risk factors	Assessment of outcome	Follow-up length	Loss to follow-up rate
												Advani et al. [18]	1	1	1	0	0	0	2	1	1	7	Good
Stevens et al. [19]	0	1	1	1	1	0	1	1	0	6	Satisfactory
Khieosanuk et al. [20]	0	0	0	1	0	0	0	1	0	2	Unsatisfactory