# Robustness in sparse linear models: relative efficiency based on robust approximate message passing

Understanding efficiency in high dimensional linear models is a longstanding problem of interest. Classical work with smaller dimensional problems dating back to Huber and Bickel has illustrated the benefits of efficient loss functions. When the number of parameters p is of the same order as the sample size n, p ≈ n, an efficiency pattern different from the one of Huber was recently established. In this work, we consider the effects of model selection on the estimation efficiency of penalized methods. In particular, we explore whether sparsity, results in new efficiency patterns when p > n. In the interest of deriving the asymptotic mean squared error for regularized M-estimators, we use the powerful framework of approximate message passing. We propose a novel, robust and sparse approximate message passing algorithm (RAMP), that is adaptive to the error distribution. Our algorithm includes many non-quadratic and non-differentiable loss functions. We derive its asymptotic mean squared error and show its convergence, while allowing p, n, s →∞, with n/p ∈ (0,1) and n/s ∈ (1,∞). We identify new patterns of relative efficiency regarding a number of penalized M estimators, when p is much larger than n. We show that the classical information bound is no longer reachable, even for light--tailed error distributions. We show that the penalized least absolute deviation estimator dominates the penalized least square estimator, in cases of heavy--tailed distributions. We observe this pattern for all choices of the number of non-zero parameters s, both s ≤ n and s ≈ n. In non-penalized problems where s =p ≈ n, the opposite regime holds. Therefore, we discover that the presence of model selection significantly changes the efficiency patterns.

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