Table 4. Statistical results of the included studies.

Groups	Reference No.	OR (95% CI)	P	Heterogeneity test
				Q	P
Current smoking	7-8,10-14,17-18,21	1.32 (0.99-1.76)	0.055	55.42	0.000
Sensitivity analysis (1)	7,11-14,21	1.16 (0.83-1.64)	0.383	23.24	0.000
Sensitivity analysis (2)	6,10-13	1.50 (1.08-2.09)	0.016	38.45	0.000
Ever smoking	9,11-12	1.12 (0.98-1.28)	0.107	0.45	0.800

Sensitivity analysis (1): Only the studies which adjusted age and gender were included; Sensitivity analysis (2): Only general population were included.