If a normal probability-probability or quantile-quantile plot for the variable x is requested, the n nonmissing values of x are first ordered from smallest to largest:
If a Q-Q plot is requested (with a PLOT statement of the form PLOT yvariable*NQQ.), the ith-ordered value is represented by a point with y-coordinate and x-coordinate , where is the standard normal distribution.
If a P-P plot is requested (with a PLOT statement of the form PLOT yvariable*NPP.), the ith-ordered value is represented by a point with y-coordinate and x-coordinate , where is the mean of the nonmissing x-values and is the standard deviation. If an x-value has multiplicity k (that is, ), then only the point is displayed.