Normal quantile plot for the ES( z ) between impairment and health-related quality of life. The plot compares two distributions by plotting there quantiles against each other. One of the distributions is the standard normal distribution (X as) while the other is the distribution one wishes to compare it against (here the Y as is defined as ES(z)i/√i). The deviations from linearity indicate deviations in normality. Publication bias will tend to leave strange gaps in the plot or lead to strange curves.