Semiparametric Pseudo-Likelihood Estimation in Markov Random Fields

International audienceProbabilistic graphical models for continuous variables can be built out of either parametric or nonparametric conditional density estimators. While several research efforts have been focusing on parametric approaches (such as Gaussian models), kernel-based estimators are still the only viable and well-understood option for nonparametric density estimation. This paper develops a semiparametric estimator of probability density functions based on the nonparanormal transformation, which has been recently proposed for mapping arbitrarily distributed data samples onto normally distributed datasets. Pointwise and uniform consistency properties are established for the developed method. The resulting density model is then applied to pseudo-likelihood estimation in Markov random fields. An experimental evaluation on data distributed according to a variety of density functions indicates that such semiparametric Markov random field models significantly outperform both their Gaussian and kernel-based alternatives in terms of prediction accuracy

Semiparametric Pseudo-Likelihood Estimation in Markov Random Fields

International audienceProbabilistic graphical models for continuous variables can be built out of either parametric or nonparametric conditional density estimators. While several research efforts have been focusing on parametric approaches (such as Gaussian models), kernel-based estimators are still the only viable and well-understood option for nonparametric density estimation. This paper develops a semiparametric estimator of probability density functions based on the nonparanormal transformation, which has been recently proposed for mapping arbitrarily distributed data samples onto normally distributed datasets. Pointwise and uniform consistency properties are established for the developed method. The resulting density model is then applied to pseudo-likelihood estimation in Markov random fields. An experimental evaluation on data distributed according to a variety of density functions indicates that such semiparametric Markov random field models significantly outperform both their Gaussian and kernel-based alternatives in terms of prediction accuracy

Two likelihood-based semiparametric estimation methods for panel count data with covariates

We consider estimation in a particular semiparametric regression model for the mean of a counting process with ``panel count'' data. The basic model assumption is that the conditional mean function of the counting process is of the form $E\{\mathbb{N}(t)|Z\}=\exp(\beta_0^TZ)\Lambda_0(t)$ where $Z$ is a vector of covariates and $\Lambda_0$ is the baseline mean function. The ``panel count'' observation scheme involves observation of the counting process $\mathbb{N}$ for an individual at a random number $K$ of random time points; both the number and the locations of these time points may differ across individuals. We study semiparametric maximum pseudo-likelihood and maximum likelihood estimators of the unknown parameters $(\beta_0,\Lambda_0)$ derived on the basis of a nonhomogeneous Poisson process assumption. The pseudo-likelihood estimator is fairly easy to compute, while the maximum likelihood estimator poses more challenges from the computational perspective. We study asymptotic properties of both estimators assuming that the proportional mean model holds, but dropping the Poisson process assumption used to derive the estimators. In particular we establish asymptotic normality for the estimators of the regression parameter $\beta_0$ under appropriate hypotheses. The results show that our estimation procedures are robust in the sense that the estimators converge to the truth regardless of the underlying counting process.Comment: Published in at http://dx.doi.org/10.1214/009053607000000181 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

Nonparametric inference in hidden Markov models using P-splines

Hidden Markov models (HMMs) are flexible time series models in which the distributions of the observations depend on unobserved serially correlated states. The state-dependent distributions in HMMs are usually taken from some class of parametrically specified distributions. The choice of this class can be difficult, and an unfortunate choice can have serious consequences for example on state estimates, on forecasts and generally on the resulting model complexity and interpretation, in particular with respect to the number of states. We develop a novel approach for estimating the state-dependent distributions of an HMM in a nonparametric way, which is based on the idea of representing the corresponding densities as linear combinations of a large number of standardized B-spline basis functions, imposing a penalty term on non-smoothness in order to maintain a good balance between goodness-of-fit and smoothness. We illustrate the nonparametric modeling approach in a real data application concerned with vertical speeds of a diving beaked whale, demonstrating that compared to parametric counterparts it can lead to models that are more parsimonious in terms of the number of states yet fit the data equally well

Generalized structured additive regression based on Bayesian P-splines

Generalized additive models (GAM) for modelling nonlinear effects of continuous covariates are now well established tools for the applied statistician. In this paper we develop Bayesian GAM's and extensions to generalized structured additive regression based on one or two dimensional P-splines as the main building block. The approach extends previous work by Lang und Brezger (2003) for Gaussian responses. Inference relies on Markov chain Monte Carlo (MCMC) simulation techniques, and is either based on iteratively weighted least squares (IWLS) proposals or on latent utility representations of (multi)categorical regression models. Our approach covers the most common univariate response distributions, e.g. the Binomial, Poisson or Gamma distribution, as well as multicategorical responses. For the first time, we present Bayesian semiparametric inference for the widely used multinomial logit models. As we will demonstrate through two applications on the forest health status of trees and a space-time analysis of health insurance data, the approach allows realistic modelling of complex problems. We consider the enormous flexibility and extendability of our approach as a main advantage of Bayesian inference based on MCMC techniques compared to more traditional approaches. Software for the methodology presented in the paper is provided within the public domain package BayesX