# The out-of-sample prediction error of the square-root-LASSO and related estimators

Joint with José Luis Montiel Olea , Cynthia Rush, and Johannes Wiesel

Abstract: We study the classical problem of predicting an outcome variable, $Y$, using a linear combination of a $d$-dimensional covariate vector, $\mathbf{X}$. We are interested in linear predictors whose coefficients solve: \begin{align*} \inf_{\boldsymbol{\beta} \in \mathbb{R}^d} \left( \mathbb{E}_{\mathbb{P}_n} \left[ \left|Y-\mathbf{X}^{\top}\beta \right|^r \right] \right)^{1/r} +\delta , \rho\left(\boldsymbol{\beta}\right), \end{align*} where $\delta>0$ is a regularization parameter, $\rho:\mathbb{R}^d\to \mathbb{R}_+$ is a convex penalty function, $\mathbb{P}_n$ is the empirical distribution of the data, and $r\geq 1$. Our main contribution is a new bound on the out-of-sample prediction error of such estimators.

The new bound is obtained by combining three new sets of results. First, we provide conditions under which linear predictors based on these estimators solve a *distributionally robust optimization* problem: they minimize the worst-case prediction error over distributions that are close to each other in a type of *max-sliced Wasserstein metric*. Second, we provide a detailed finite-sample and asymptotic analysis of the statistical properties of the balls of distributions over which the worst-case prediction error is analyzed. Third, we present an oracle recommendation for the choice of regularization parameter, $\delta$, that guarantees good out-of-sample prediction error.