Pooled ORs and 95% CIs of the overall and stratified meta-analyses.

OR: odds ratio; CI: confidence interval. The results were in bold, if the 95% CI excluded 1 or P<0.05.
athe statistical significance of the pooled OR was determined by the Z test;
bNo statistical significance was found by the heterogeneity test, then the fixed-effects model was adopted here.
cconfirmatory analyses with a random-effect model was used if there was significant heterogeneity.
dwhen the dominant model was analyzed, the data studied by Wenzhang Wang (2011) was added.