2.4 Statistical analysis
We conducted the review using the Stata 12.0 and RevMan 5.3 software. Comparisons of the dichotomous variables were performed using odds ratio (OR) with 95% confidence interval (CI). For continuous variables, the standard mean differences (Std. MD) with 95% CI were used. Z-test was performed to determine the statistical significance of results with statistical significance defined as P<0.05 . Heterogeneity between articles was assessed by Cochrane’s Q-test. Data with low heterogeneity (P≥0.10 andI2<50% ) were analyzed by a fixed-effects model while a random-effect model was used for data with high heterogeneity (P<0.10 andI2>50% ). To explore sources of heterogeneity the leave-one-out sensitivity analysis was also conducted. Furthermore, a funnel plot was used to evaluate the publication bias when applicable.