Double P-P Plots for Comparing Differences Between Two Groups

Abstract

This article suggests a graphic technique that uses P-P plots to show the extent to which two groups differ on two variables. It can be used even if the variables are measured in completely different, noncomparable units. The comparison is symmetric with respect to the variables and the groups. It reflects the differences between the groups over the full range of values of each variable. Two examples from educational testing are included; the technique is also applicable to other fields of inquiry.