Heterogeneity between studies will be assessed using the Cochran's Q and Higgins I2 statistic. A P-value of < .10 for the Chi2 statistic or an I2 > 50% will be considered as showing considerable heterogeneity, and data will be analyzed using the Mantel–Haenszel random-effect model. Under other circumstances, we will apply the Mantel–Haenszel fixed-effect model.