Research Assistant, Ph D Student
Research ScientistFacebookJune 2018
Bayes High-Dimensional Density Estimation Using Multiscale Dictionaries
Although Bayesian density estimation using discrete mixtures has good performance in modest dimensions, there is a lack of statistical and computational scalability to high-dimensional multivariate cases. To combat the curse of dimensionality, it is necessary to assume the data are concentrated near a lower-dimensional subspace. However, Bayesian methods for learning this subspace along with the density of the data scale poorly computationally. To solve this problem, we propose an empirical Bayes approach, which estimates a multiscale dictionary using geometric multiresolution analysis in a first stage. We use this dictionary within a multiscale mixture model, which allows uncertainty in component allocation, mixture weights and scaling factors over a binary tree. A computational algorithm is proposed, which scales efficiently to massive dimensional problems. We provide some theoretical support for this method, and illustrate the performance through simulated and real data examples.
Bayesian Computation for High-Dimensional Continuous & Sparse Count Data
Probabilistic modeling of multidimensional data is a common problem in practice. When data are continuous, one common approach is to suppose that the observed data are close to a lower-dimensional smooth manifold. There are a rich variety of manifold learning methods available, which allow mapping of data points to the manifold. However, there is a clear lack of probabilistic methods that allow learning of the manifold along with the generative distribution of the observed data. The best attempt is the Gaussian process latent variable model (GP-LVM), but identifiability issues lead to poor performance. We solve these issues by proposing a novel Coulomb repulsive process (Corp) for locations of points on the manifold, inspired by physical models of electrostatic interactions among particles. Combining this process with a GP prior for the mapping function yields a novel electrostatic GP (electroGP) process. Another popular approach is to suppose that the observed data are closed to one or a union of lower-dimensional linear subspaces. However, popular methods such as probabilistic principal component analysis scale poorly computationally. We introduce a novel empirical Bayesian method that we term geometric density estimation (GEODE), which assumes the data is centered near a low-dimensional linear subspace. We show that, with mild assumptions on the prior, the subspace spanned by the principal axes of the data maximizes the posterior mode. Hence, leveraged on the geometric information of the data, GEODE easily scales to massive dimensional problems. It is also capable of learning the intrinsic dimension via a novel shrinkage prior. Finally, we mix GEODE across a dyadic clustering tree to account for nonlinear cases. When data are discrete, a common strategy is to define a generalized linear model (GLM) for each variable, with dependence in the different variables induced through including multivariate latent variables in the GLMs. The Bayesian inference for these models usually rely on data augmented Markov chain Monte Carlo (DA-MCMC) method, which has a provable slow mixing rate when the data is imbalanced. For more scalable inference, we propose Bayesian mosaic, a parallelizable composite posterior, for scalable Bayesian inference on a broad class of multivariate discrete data models. Sampling is embarrassingly parallel since Bayesian mosaic is a multiplication of component posteriors that can be independently sampled from. Analogous to composite likelihood methods, these component posteriors are based on univariate or bivariate marginal densities. Utilizing the fact that the score functions of these densities are unbiased, we show that Bayesian mosaic is consistent and asymptotically normal under mild conditions. Since the evaluation of univariate or bivariate marginal densities can rely on numerical integration, sampling from Bayesian mosaic bypasses the traditional data augmented Markov chain Monte Carlo (DA-MCMC) method, which has a provably slow mixing rate when data are imbalanced. Moreover, we show that sampling from Bayesian mosaic has better scalability to large sample size than DA-MCMC. The performance of the proposed methods and models will be demonstrated via both simulation studies and real-world applications.