Table 2.

Quality assessment of the included cohort trials

	Study (year)
	Sang-Mok Kim (2014)	Chao-Wei Tu (2014)	Qirui Ding (2017)
Selection
Representativeness of the exposed cohort	*	*	*
Ascertainment of exposure	*	*	*
Outcome not present at the start of the study	*	*	*
Comparability
Comorbidities	*
Other factors	*	*	*
Outcome
Assessment of the outcome	*	*	*
Follow-up long enough for the outcome to occur	*	*	*
Adequacy of the follow-up	*	*	*
Total	8	7	7