It should be noted that the importance of the observed value of I² depends on the magnitude and direction of the effects and the strength of the evidence for heterogeneity. We regard heterogeneity as substantial if the I² was greater than 50% or the P value for the Chi2 test for heterogeneity is less than 0.10. If we find considerable levels of heterogeneity (75% or higher), we will explore the forest plot to identify studies that contribute to the greatest heterogeneity