Regularized Linear Regression for Binary Classification
Danil Akhtiamov Reza Ghane Babak Hassibi
Regularized linear regression is a promising approach for binary
classification problems in which the training set has noisy labels since the
regularization term can help to avoid interpolating the mislabeled data points.
In this paper we provide a systematic study of the effects of the
regularization strength on the performance of linear classifiers that are
trained to solve binary classification problems by minimizing a regularized
least-squares objective. We consider the over-parametrized regime and assume
that the classes are generated from a Gaussian Mixture Model (GMM) where a
fraction $c<\frac{1}{2}$ of the training data is mislabeled. Under these
assumptions, we rigorously analyze the classification errors resulting from the
application of ridge, $\ell_1$, and $\ell_\infty$ regression. In particular, we
demonstrate that ridge regression invariably improves the classification error.
We prove that $\ell_1$ regularization induces sparsity and observe that in many
cases one can sparsify the solution by up to two orders of magnitude without
any considerable loss of performance, even though the GMM has no underlying
sparsity structure. For $\ell_\infty$ regularization we show that, for large
enough regularization strength, the optimal weights concentrate around two
values of opposite sign. We observe that in many cases the corresponding
"compression" of each weight to a single bit leads to very little loss in
performance. These latter observations can have significant practical
ramifications.