Positions and QQ Plots

Quantile–quantile (QQ) plots for comparing two distributions are constructed by matching like-positioned values (i.e., quantiles) in the two distributions. These plots can reveal outliers, differences in location and scale, and other differences between the distributions. A particularly useful application is comparing residuals from an estimated linear model to the normal. A robust estimate, such as the Sen–Theil estimate, of the regression line is important. Extensions to two-dimensional QQ plots are presented, relying on a particular notion of multivariate position.