2.6 Bias detection
All statistics were analyzed using Stata 12.0 version (Stata Corporation, College Station, TX, United States). Funnel plot is a method to identify the existence of publication bias by visual observation. This method takes the effect size as the abscissa and the y-coordinate is the standard error. The dispersion of small samples is large, so it is often at the bottom of the funnel plot, while that of large samples is small, so it is at the top.