Visualizing Tests for Equality of Covariance Matrices

This paper explores a variety of topics related to the question of testing the equality of covariance matrices in multivariate linear models, particularly in the MANOVA setting. The main focus is on graphical methods that can be used to address the evaluation of this assumption. We introduce some extensions of data ellipsoids, hypothesis-error (HE) plots and canonical discriminant plots and demonstrate how they can be applied to the testing of equality of covariance matrices. Further, a simple plot of the components of Box’s M test is proposed that shows how groups differ in covariance and also suggests other visualizations and alternative test statistics. These methods are implemented and freely available in the heplots and candisc packages for R. Examples from the paper are available in supplementary materials.