Delta Plot

A delta plot is a graphical display for comparing two distributions that is closely related to quantile-quantile (QQ) plots. A common application in education is to inspect whether there is differential item functioning by plotting distributions of item discriminability in one population versus that of another. The setup is one with two populations (or conditions), which are generically called Population A and Population B. Let X(p) and Y(p) denote the pth percentile for the dependent measure for Populations A and B, respectively; let d(p) = Y(p) − X(p) be the difference, or the effect, at the pth percentile; and let m(p) be the average effect at the pth percentile. The delta plot is a plot of the effect, d(p), as a function of the average, ...

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