7. Variational Bayes for Hierarchical Mixture Models

In recent years, sparse classification problems have emerged in many fields of study. Finite mixture models have been developed to facilitate Bayesian inference where parameter sparsity is substantial. Classification with finite mixture models is based on the posterior expectation of latent indicator variables. These quantities are typically estimated using the expectation-maximization (EM) algorithm in an empirical Bayes approach or Markov chain Monte Carlo (MCMC) in a fully Bayesian approach. MCMC is limited in applicability where high-dimensional data are involved because its sampling-based nature leads to slow computations and hard-to-monitor convergence. In this chapter, we investigate the feasibility and performance of variational Bayes (VB) approximation in a fully Bayesian framework. We apply the VB approach to fully Bayesian versions of several finite mixture models that have been proposed in bioinformatics, and find that it achieves desirable speed and accuracy in sparse classification with finite mixture models for high-dimensional data.