In this paper, we propose a general algorithm to sample sequentially from a sequence of probability distributions known up to a normalizing constant and de ned on a common space. A sequence of increasingly large arti cial joint distributions is built; each of these distributions admits a marginal which is a distribution of interest. To sample from these distributions, we use sequential Monte Carlo methods. We show that these methods can be interpreted as interacting particle approximations of a nonlinear Feynman-Kac ow in distribution space. One interpretation of the Feynman-Kac ow corresponds to a nonlinear Markov kernel admitting a speci ed invariant distribution and is a natural nonlinear extension of the standard Metropolis-Hastings algorithm. Many theoretical results have already been established for such ows and their particle approximations. We demonstrate the use of these algorithms through simulation.

Finding people in pictures presents a particularly difficult object recognition problem. We show how to find people by finding candidate body segments, and then constructing assemblies of segments that are consistent with the constraints on the appearance of a person that result from kinematic properties. Since a reasonable model of a person requires at least nine segments, it is not possible to inspect every group, due to the huge combinatorial complexity. We propose two

We introduce a new method for estimating recombination rates from population genetic data. The method uses a computationally-intensive statistical procedure (importance sampling) to calculate the likelihood under a coalescent-based model. Detailed comparisons of the new algorithm with two existing methods (one based on importance sampling and one based on MCMC) show it to be substantially more efficient. (The improvement over the existing importance sampling scheme is typically by four orders of magnitude.) The existing approaches not infrequently led to misleading results on the problems we investigated. We also performed a simulation study to look at the properties of the maximum likelihood estimator (mle) of the recombination rate, and its robustness to misspecification of the demographic model.

Optimal estimation problems for non-linear non-Gaussian state-space models do not typically admit analytic solutions. Since their introduction in 1993, particle filtering methods have become a very popular class of algorithms to solve these estimation problems numerically in an online manner, i.e. recursively as observations become available, and are now routinely used in fields as diverse as computer vision, econometrics, robotics and navigation. The objective of this tutorial is to provide a complete, up-to-date survey of this field as of 2008. Basic and advanced particle methods for filtering as well as smoothing are presented.

This paper is concerned with recursively estimating the internal state of a nonlinear dynamic system by processing noisy measurements and the known system input. In the case of continuous states, an exact analytic representation of the probability density characterizing the estimate is generally too complex for recursive estimation or even impossible to obtain. Hence, it is replaced by a convenient type of approximate density characterized by a finite set of parameters. Of course, parameters are desired that systematically minimize a given measure of deviation between the (often unknown) exact density and its approximation, which in general leads to a complicated optimization problem. Here, a new framework for state estimation based on progressive processing is proposed. Rather than trying to solve the original problem, it is exactly converted into a corresponding system of explicit ordinary first–order differential equations. Solving this system over a finite “time” interval yields the desired optimal density parameters.

Gaussian process priors can be used to define flexible, probabilistic classification models. Unfortunately exact Bayesian inference is analytically intractable and various approximation techniques have been proposed. In this work we review and compare Laplace’s method and Expectation Propagation for approximate Bayesian inference in the binary Gaussian process classification model. We present a comprehensive comparison of the approximations, their predictive performance and marginal likelihood estimates to results obtained by MCMC sampling. We explain theoretically and corroborate empirically the advantages of Expectation Propagation compared to Laplace’s method. Keywords: Gaussian process priors, probabilistic classification, Laplace’s approximation, expectation propagation, marginal likelihood, evidence, MCMC

Sequential random sampling (`Markov Chain Monte-Carlo') is a popular strategy for many vision problems involving multimodal distributions over high-dimensional parameter spaces. It applies both to importance sampling (where one wants to sample points according to their `importance ' for some calculation, but otherwise fairly) and to global optimization (where one wants to find good minima, or at least good starting points for local minimization, regardless of fairness) . Unfortunately, most sequential samplers are very prone to becoming `trapped' for long periods in unrepresentative local minima, which leads to biased or highly variable estimates. We present a general strategy for reducing MCMC trapping that generalizes Voter's `hyperdynamic sampling' from computational chemistry. The local gradient and curvature of the input distribution are used to construct an adaptive importance sampler that focuses samples on low cost negative curvature regions likely to contain `transition states' --- codimension-1 saddle points representing `mountain passes' connecting adjacent cost basins. This substantially accelerates inter-basin transition rates while still preserving correct relative transition probabilities. Experimental tests on the difficult problem of 3D articulated human pose estimation from monocular images show significantly enhanced minimum exploration.

Deep Belief Networks (DBN’s) are generative models that contain many layers of hidden variables. Efficient greedy algorithms for learning and approximate inference have allowed these models to be applied successfully in many application domains. The main building block of a DBN is a bipartite undirected graphical model called a restricted Boltzmann machine (RBM). Due to the presence of the partition function, model selection, complexity control, and exact maximum likelihood learning in RBM’s are intractable. We show that Annealed Importance Sampling (AIS) can be used to efficiently estimate the partition function of an RBM, and we present a novel AIS scheme for comparing RBM’s with different architectures. We further show how an AIS estimator, along with approximate inference, can be used to estimate a lower bound on the log-probability that a DBN model with multiple hidden layers assigns to the test data. This is, to our knowledge, the first step towards obtaining quantitative results that would allow us to directly assess the performance of Deep Belief Networks as generative models of data. 1.

this paper we will focus on an alternative class of filters in which theoretical distributions on the state space are approximated by simulated random measures. The first goal in filter design is to produce a compact description of the posterior distribution of the state given all the observations available so far. A basic requirement is that this description should be readily updated as new data become available. A mechanism has therefore to be devised which enables the approximating random measure to evolve and adapt. 3 SIMULATION BASED FILTERS Simulation based filters have a long history in the engineering literature, dating back to the work of Handschin and Mayne (1969); Handschin (1970); Akashi and Kumamoto (1977). Doucet (1998) provides a comprehensive review of the material. Since the Kalman filter is essentially a Bayesian update formula, the theory of Bayesian time series analysis is directly relevant (West and Harrison, 1997). We take as our starting point the filter developed by Gordon (1993); Gordon et al. (1993). The essence of the method is contained in a paper by Rubin (1988) who proposed the Sampling Importance Resampling (SIR) algorithm for obtaining samples from a complex posterior distribution without recourse to MCMC. In the simple non-dynamic case described by Rubin (1988), the method consists of sampling n observations from the prior distribution, attaching weights to the sampled points according to their likelihood, and then sampling with replacement from this weighted discrete distribution. As n ! 1, the resulting set of values then approximates a sample from the required posterior (Smith and Gelfand, 1992). In the dynamic version, proposed by Gordon et al. (1993), the SIR algorithm is applied repeatedly as new data are acquired. One can think of...

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