Table 1.

Quality assessment by Newcastle–Ottawa scale.

Study	Representativeness of the exposed cohort	Selection of the nonexposed cohort	Ascertainment of exposure	Outcome of interest was not present at start of study	Comparability of cohorts on the basis of the design or analysis	Assessment of outcome	Follow-up was long enough	Adequacy of follow-up of cohorts	Total quality score
Lei et al. [21]	1	1	1	1	0	1	1	0	6
Kim et al. [9]	1	1	1	1	0	1	1	0	6
Suzuki et al. [10]	1	1	1	1	0	1	1	1	7
Wang et al. [11]	1	1	1	1	2	1	1	0	8
Y Ito 2012 (1)	1	1	1	1	0	1	1	1	7
Y Ito 2012 (2)	1	1	1	1	0	1	1	1	7
Kwong et al. [12]	1	1	1	1	0	1	1	0	6
Liu et al. [13]	1	1	1	1	2	1	1	1	9
Kim et al. [14]	1	1	1	1	0	1	1	1	7
Wang et al. [6]	1	1	1	1	2	1	1	0	8
Lim et al. [15]	1	1	1	1	0	1	1	0	6
Liang et al. [16]	1	1	1	1	0	1	1	0	6
Yang et al. [17]	1	1	1	1	0	1	1	1	7
Zhang et al. [23]	1	1	1	1	0	1	1	1	7
Liang et al. [18]	1	1	1	1	0	1	1	1	7