2.5 Statistical analysis
Binary variables were expressed as odds ratios (ORs) with 95% confidence intervals (CIs). Continuous variables were analyzed using the mean difference (MD) and the corresponding 95% CI estimated using the inverse-variance method. A two-sided P-value≤0.05 was considered statistically significant. The fixed-effects model and the random-effects model were considered based on the level of heterogeneity. The heterogeneity of studies was evaluated by Cochran’s Q and the I2 statistic. I2 lies between 0% and 100% with larger values showing increasing heterogeneity. I2 value >50% was considered high degrees of heterogeneity and the random model was used in the subgroup analysis or sensitivity analysis excluding the trials that potentially biased the results to avoid publication bias, otherwise, a fixed-effects model was used 13,14. We performed sensitivity analysis by omitting one study successively to evaluate the impact of the individual studies on the pooled effect size. All statistical analyses were performed using the RevMan version 5.3 (Nordic Cochrane Center; The Cochrane Collaboration, 2014).