a: RCTs; b: non-RCTs. The 95% confidence interval (CI) for the hazard ratio for each study is represented by a horizontal line and the point estimate is represented by a square. The size of the square corresponds to the weight of the study in the meta-analysis. The 95% CI for pooled estimates is represented by a diamond. Data for a fixed-effects model are shown as there was no statistical heterogeneity. df = degrees of freedom; I^{2} = percentage of the total variation across studies due to heterogeneity; IV = Inverse Variance; SE = standard error; Z = test of overall treatment effect.