ASYMPTOTIC RISK AND PHASE TRANSITION OF l1-PENALIZED ROBUST ESTIMATOR

Mean square error (MSE) of the estimator can be used to evaluate the performance of a regression model. In this paper, we derive the asymptotic MSE of l(1)-penalized robust estimators in the limit of both sample size n and dimension p going to infinity with fixed ratio n/p -> delta. We focus on the l(1)-penalized least absolute deviation and l(1)-penalized Huber's regressions. Our analytic study shows the appearance of a sharp phase transition in the two-dimensional sparsity-undersampling phase space. We derive the explicit formula of the phase boundary. Remarkably, the phase boundary is identical to the phase transition curve of LASSO which is also identical to the previously known Donoho-Tanner phase transition for sparse recovery. Our derivation is based on the asymptotic analysis of the generalized approximation passing (GAMP) algorithm. We establish the asymptotic MSE of the l(1)-penalized robust estimator by connecting it to the asymptotic MSE of the corresponding GAMP estimator. Our results provide some theoretical insight into the high-dimensional regression methods. Extensive computational experiments have been conducted to validate the correctness of our analytic results. We obtain fairly good agreement between theoretical prediction and numerical simulations on finite-size systems.