Quantile tomography: using quantiles with multivariate data

The use of quantiles to obtain insights about multivariate data is ad- dressed. It is argued that incisive insights can be obtained by considering direc- tional quantiles, the quantiles of projections. Directional quantile envelopes are proposed as a way to condense this kind of information; it is demonstrated that they are essentially halfspace (Tukey) depth levels sets, coinciding for elliptic distri- butions (in particular multivariate normal) with density contours. Relevant ques- tions concerning their indexing, the possibility of the reverse retrieval of directional quantile information, invariance with respect to affine transformations, and approx- imation/asymptotic properties are studied. It is argued that analysis in terms of directional quantiles and their envelopes offers a straightforward probabilistic inter- pretation and thus conveys a concrete quantitative meaning; the directional defini- tion can be adapted to elaborate frameworks, like estimation of extreme quantiles and directional quantile regression, the regression of depth contours on covariates. The latter facilitates the construction of multivariate growth charts—the question that motivated this development.