Table 2.

Quality evaluation of studies.

	Selection				Comparability	Outcome
	Representativeness of the exposed cohort	Selection of nonexposure group	Exposure confirmation	Outcome events before the start of the study	Comparability between research design and structure	assessment of outcome	was follow-up long enough for outcomes to occur	adequacy of follow-up of cohort	Total
Blair CK[16]	0	1	0	1	2	1	1	1	7
Wernli KJ[17]	0	1	0	1	2	1	1	1	7
Li Y[18]	0	1	0	1	2	1	1	0	7
Fraser DM[11]	0	1	1	1	2	1	1	1	8
Barron TI[19]	0	1	1	1	2	1	1	0	7
Barron TI[20]	0	1	1	1	2	1	1	1	8
Bradley MC[21]	0	1	0	1	2	1	1	1	7
Shiao J[22]	0	1	1	1	0	1	1	1	6
Mc Menamin MC[23]	0	1	1	1	2	1	1	0	7
Wang T[24]	0	1	0	1	2	1	1	0	6
Frisk G[25]	0	1	1	1	2	1	1	1	8
Cronin-Fenton DP[26]	0	1	1	1	2	1	1	0	7
Bens A[27]	0	1	1	1	2	1	1	1	8