On the sign recovery by LASSO, thresholded LASSO and thresholded Basis Pursuit Denoising
Résumé
Basis Pursuit (BP), Basis Pursuit DeNoising (BPDN), and LASSO are popular methods for identifying
important predictors in the high-dimensional linear regression model Y = Xβ + ε. By definition, when
ε = 0, BP uniquely recovers β when Xβ = Xb and β different than b implies L1 norm of β is smaller than the L1 norm
of b (identifiability condition). Furthermore, LASSO can recover the sign of β only under a much stronger
irrepresentability condition. Meanwhile, it is known that the model selection properties of LASSO can be improved by hard-thresholding
its estimates. This article supports these findings by proving that thresholded LASSO, thresholded BPDN
and thresholded BP recover the sign of β in both the noisy and noiseless cases if and only if β is identifiable
and large enough. In particular, if X has iid Gaussian entries and the number of predictors grows linearly
with the sample size, then these thresholded estimators can recover the sign of β when the signal sparsity is
asymptotically below the Donoho-Tanner transition curve. This is in contrast to the regular LASSO, which
asymptotically, recovers the sign of β only when the signal sparsity tends to 0. Numerical experiments show
that the identifiability condition, unlike the irrepresentability condition, does not seem to be affected by the
structure of the correlations in the X matrix.
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