Dr. Christopher Hans; Associate Professor, Department of Statistics, Ohio State University

Abstract:

Penalized optimization procedures are sometimes described as Bayesian approaches to parameter estimation, prediction and variable selection.  In this talk I examine this connection in the context of the lasso (Tibshirani, 1996) and the elastic net (Zou and Hastie, 2005). Motivated by the proliferation of these (and related) approaches to penalized optimization, I introduce a new class of prior distributions --- the "orthant normal" distribution --- for the regression coefficients in a Bayesian regression model and show that this prior gives rise to the lasso and elastic net point estimates as the posterior mode.  By providing a complete characterization of the prior distribution, I allow for model-based inference that moves beyond exclusive use of the posterior mode, including coherent Bayesian prediction and formal Bayesian model comparison.  In contrast to penalized optimization procedures, where values for the penalty parameters are often selected via cross validation, the Bayesian approach allows for uncertainty about these parameters to be explicitly  included in the model.  I show that the orthant normal distribution has a scale-mixture of  normals representation, providing additional insight into the particular form of shrinkage employed by the elastic net.  Posterior inference is shown to be easily achieved via MCMC methods.   Finally, I generalize the basic orthant normal distribution to allow for a priori dependence in the regression coefficients, yielding a framework for constructing new penalties.  In particular I discuss an approach for incorporating prior information about dependence structure in the covariates that resembles Zellner's g-prior but that also allows for lasso-like shrinkage.