Reducing Subspace Models for Large-Scale Covariance Regression

TL;DR

This paper introduces a novel envelope model for joint mean and covariance regression in high-dimensional, small-sample settings, focusing on covariance heterogeneity and mean-level differences.

Contribution

It develops a new method using Monte Carlo EM and MCMC to identify low-dimensional subspaces explaining mean and covariance differences, with application to metabolomics of aging.

Findings

Effective identification of covariance heterogeneity

Application to aging metabolomics data

Provides R code for model development

Abstract

We develop an envelope model for joint mean and covariance regression in the large $p$, small $n$ setting. In contrast to existing envelope methods, which improve mean estimates by incorporating estimates of the covariance structure, we focus on identifying covariance heterogeneity by incorporating information about mean-level differences. We use a Monte Carlo EM algorithm to identify a low-dimensional subspace which explains differences in both means and covariances as a function of covariates, and then use MCMC to estimate the posterior uncertainty conditional on the inferred low-dimensional subspace. We demonstrate the utility of our model on a motivating application on the metabolomics of aging. We also provide R code which can be used to develop and test other generalizations of the response envelope model.