# Cross validation in sparse linear regression with piecewise continuous nonconvex penalties and its acceleration

## TL;DR

This paper analyzes the performance of sparse linear regression with nonconvex penalties, especially SCAD, using theoretical and numerical methods, and introduces an efficient cross-validation error approximation and an annealing procedure.

## Contribution

It provides a theoretical analysis of SCAD penalties using the replica method, develops an efficient cross-validation error approximation, and proposes a nonconvexity annealing method for solution path computation.

## Key findings

- SCAD outperforms L1 in a wide parameter range.
- The approximate cross-validation formula is effective in the unique solution region.
- The nonconvexity annealing method efficiently traces the solution path.

## Abstract

We investigate the signal reconstruction performance of sparse linear regression in the presence of noise when piecewise continuous nonconvex penalties are used. Among such penalties, we focus on the SCAD penalty. The contributions of this study are three-fold: We first present a theoretical analysis of a typical reconstruction performance, using the replica method, under the assumption that each component of the design matrix is given as an independent and identically distributed (i.i.d.) Gaussian variable. This clarifies the superiority of the SCAD estimator compared with $\ell_1$ in a wide parameter range, although the nonconvex nature of the penalty tends to lead to solution multiplicity in certain regions. This multiplicity is shown to be connected to replica symmetry breaking in the spin-glass theory. We also show that the global minimum of the mean square error between the estimator and the true signal is located in the replica symmetric phase. Second, we develop an approximate formula efficiently computing the cross-validation error without actually conducting the cross-validation, which is also applicable to the non-i.i.d. design matrices. It is shown that this formula is only applicable to the unique solution region and tends to be unstable in the multiple solution region. We implement instability detection procedures, which allows the approximate formula to stand alone and resultantly enables us to draw phase diagrams for any specific dataset. Third, we propose an annealing procedure, called nonconvexity annealing, to obtain the solution path efficiently. Numerical simulations are conducted on simulated datasets to examine these results to verify the theoretical results consistency and the approximate formula efficiency. Another numerical experiment on a real-world dataset is conducted; its results are consistent with those of earlier studies using the $\ell_0$ formulation.