Performance of Bayesian linear regression in a model with mismatch

For a model of high-dimensional linear regression with random design, we analyze the performance of an estimator given by the mean of a log-concave Bayesian posterior distribution with gaussian prior. The model is mismatched in the following sense: like the model assumed by the statistician, the labels-generating process is linear in the input data, but both the classifier ground-truth prior and gaussian noise variance are unknown to her. This inference model can be rephrased as a version of the Gardner model in spin glasses and, using the cavity method, we provide fixed point equations for various overlap order parameters, yielding in particular an expression for the mean-square reconstruction error on the classifier (under an assumption of uniqueness of solutions). As a direct corollary we obtain an expression for the free energy. Similar models have already been studied by Shcherbina and Tirozzi and by Talagrand, but our arguments are more straightforward and some assumptions are relaxed. An interesting consequence of our analysis is that in the random design setting of ridge regression, the performance of the posterior mean is independent of the noise variance (or "temperature") assumed by the statistician, and matches the one of the usual (zero temperature) ridge estimator.

Strong replica symmetry for high-dimensional disordered log-concave Gibbs measures

We consider a generic class of log-concave, possibly random, (Gibbs) measures. Using a new type of perturbation we prove concentration of an infinite family of order parameters called multioverlaps. These completely parametrise the quenched Gibbs measure of the system, so that their self-averaging behavior implies a simple representation of asymptotic Gibbs measures, as well as decoupling of the variables at hand in a strong sense. Our concentration results may prove themselves useful in several contexts. In particular in machine learning and high-dimensional inference, log-concave measures appear in convex empirical risk minimisation, maximum a-posteriori inference or M-estimation. We believe that our results may be applicable in establishing some type of "replica symmetric formulas" for the free energy, inference or generalisation error in such settings.