72,279 research outputs found
A decision-theoretic approach for segmental classification
This paper is concerned with statistical methods for the segmental
classification of linear sequence data where the task is to segment and
classify the data according to an underlying hidden discrete state sequence.
Such analysis is commonplace in the empirical sciences including genomics,
finance and speech processing. In particular, we are interested in answering
the following question: given data and a statistical model of
the hidden states , what should we report as the prediction under
the posterior distribution ? That is, how should you make a
prediction of the underlying states? We demonstrate that traditional approaches
such as reporting the most probable state sequence or most probable set of
marginal predictions can give undesirable classification artefacts and offer
limited control over the properties of the prediction. We propose a decision
theoretic approach using a novel class of Markov loss functions and report
via the principle of minimum expected loss (maximum expected
utility). We demonstrate that the sequence of minimum expected loss under the
Markov loss function can be enumerated exactly using dynamic programming
methods and that it offers flexibility and performance improvements over
existing techniques. The result is generic and applicable to any probabilistic
model on a sequence, such as Hidden Markov models, change point or product
partition models.Comment: Published in at http://dx.doi.org/10.1214/13-AOAS657 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A discussion on hidden Markov models for life course data
This is an introduction on discrete-time Hidden Markov models (HMM)
for longitudinal data analysis in population and life course studies. In the Markovian
perspective, life trajectories are considered as the result of a stochastic process
in which the probability of occurrence of a particular state or event depends on the
sequence of states observed so far. Markovian models are used to analyze the transition
process between successive states. Starting from the traditional formulation
of a first-order discrete-time Markov chain where each state is liked to the next
one, we present the hidden Markov models where the current response is driven
by a latent variable that follows a Markov process. The paper presents also a simple
way of handling categorical covariates to capture the effect of external factors
on the transition probabilities and existing software are briefly overviewed. Empirical
illustrations using data on self reported health demonstrate the relevance of the
different extensions for life course analysis
Hidden Markov model technique for dynamic spectrum access
Dynamic spectrum access is a paradigm used to access the spectrum dynamically. A hidden Markov model (HMM) is one in which you observe a sequence of emissions, but do not know the sequence of states the model went through to generate the emissions. Analysis of hidden Markov models seeks to recover the sequence of states from the observed data. In this paper, we estimate the occupancy state of channels using hidden Markov process. Using Viterbi algorithm, we generate the most likely states and compare it with the channel states. We generated two HMMs, one slowly changing and another more dynamic and compare their performance. Using the Baum-Welch algorithm and maximum likelihood algorithm we calculated the estimated transition and emission matrix, and then we compare the estimated states prediction performance of both the methods using stationary distribution of average estimated transition matrix calculated by both the methods
Quick Adaptive Ternary Segmentation: An Efficient Decoding Procedure For Hidden Markov Models
Hidden Markov models (HMMs) are characterized by an unobservable (hidden)
Markov chain and an observable process, which is a noisy version of the hidden
chain. Decoding the original signal (i.e., hidden chain) from the noisy
observations is one of the main goals in nearly all HMM based data analyses.
Existing decoding algorithms such as the Viterbi algorithm have computational
complexity at best linear in the length of the observed sequence, and
sub-quadratic in the size of the state space of the Markov chain. We present
Quick Adaptive Ternary Segmentation (QATS), a divide-and-conquer procedure
which decodes the hidden sequence in polylogarithmic computational complexity
in the length of the sequence, and cubic in the size of the state space, hence
particularly suited for large scale HMMs with relatively few states. The
procedure also suggests an effective way of data storage as specific cumulative
sums. In essence, the estimated sequence of states sequentially maximizes local
likelihood scores among all local paths with at most three segments. The
maximization is performed only approximately using an adaptive search
procedure. The resulting sequence is admissible in the sense that all
transitions occur with positive probability. To complement formal results
justifying our approach, we present Monte-Carlo simulations which demonstrate
the speedups provided by QATS in comparison to Viterbi, along with a precision
analysis of the returned sequences. An implementation of QATS in C++ is
provided in the R-package QATS and is available from GitHub
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Hidden Markov model and financial application
A Hidden Markov model (HMM) is a statistical model in which the system being modeled is assumed to be a Markov process with numerous unobserved (hidden) states. This report applies HMM to financial time series data to explore the underlying regimes that can be predicted by the model. These underlying regimes can be used as an important signal of market environments and used as guidance by investors to adjust their portfolio to maximize the performance. This report is composed of three chapters. The 1st chapter will introduce the difficulties in predicting financial time series, the limitations with traditional time series models, justification for choosing HMM and previous studies. The 2nd chapter will go through a detailed overview of HMM model, including the basic math frame works, and fundamental questions and algorithm to be addressed by the model. In the 3rd chapter, the trend analysis of the stock market is found using Hidden Markov Model. For a given observation sequence, the hidden sequence of states and their corresponding probability values are found. This analysis builds a platform for investors to decision makers to make decisions on the basis of probability and pattern of transition of each hidden state which cannot be observed from market data.Statistic
Hidden Markov models and neural networks for speech recognition
The Hidden Markov Model (HMMs) is one of the most successful modeling approaches for acoustic events in speech recognition, and more recently it has proven useful for several problems in biological sequence analysis. Although the HMM is good at capturing the temporal nature of processes such as speech, it has a very limited capacity for recognizing complex patterns involving more than first order dependencies in the observed data sequences. This is due to the first order state process and the assumption of state conditional independence between observations. Artificial Neural Networks (NNs) are almost the opposite: they cannot model dynamic, temporally extended phenomena very well, but are good at static classification and regression tasks. Combining the two frameworks in a sensible way can therefore lead to a more powerful model with better classification abilities. The overall aim of this work has been to develop a probabilistic hybrid of hidden Markov models and neural networks and ..
Efficient duration modelling in the hierarchical hidden semi-Markov models and their applications
Modeling patterns in temporal data has arisen as an important problem in engineering and science. This has led to the popularity of several dynamic models, in particular the renowned hidden Markov model (HMM) [Rabiner, 1989]. Despite its widespread success in many cases, the standard HMM often fails to model more complex data whose elements are correlated hierarchically or over a long period. Such problems are, however, frequently encountered in practice. Existing efforts to overcome this weakness often address either one of these two aspects separately, mainly due to computational intractability. Motivated by this modeling challenge in many real world problems, in particular, for video surveillance and segmentation, this thesis aims to develop tractable probabilistic models that can jointly model duration and hierarchical information in a unified framework. We believe that jointly exploiting statistical strength from both properties will lead to more accurate and robust models for the needed task. To tackle the modeling aspect, we base our work on an intersection between dynamic graphical models and statistics of lifetime modeling. Realizing that the key bottleneck found in the existing works lies in the choice of the distribution for a state, we have successfully integrated the discrete Coxian distribution [Cox, 1955], a special class of phase-type distributions, into the HMM to form a novel and powerful stochastic model termed as the Coxian Hidden Semi-Markov Model (CxHSMM). We show that this model can still be expressed as a dynamic Bayesian network, and inference and learning can be derived analytically.Most importantly, it has four superior features over existing semi-Markov modelling: the parameter space is compact, computation is fast (almost the same as the HMM), close-formed estimation can be derived, and the Coxian is flexible enough to approximate a large class of distributions. Next, we exploit hierarchical decomposition in the data by borrowing analogy from the hierarchical hidden Markov model in [Fine et al., 1998, Bui et al., 2004] and introduce a new type of shallow structured graphical model that combines both duration and hierarchical modelling into a unified framework, termed the Coxian Switching Hidden Semi-Markov Models (CxSHSMM). The top layer is a Markov sequence of switching variables, while the bottom layer is a sequence of concatenated CxHSMMs whose parameters are determined by the switching variable at the top. Again, we provide a thorough analysis along with inference and learning machinery. We also show that semi-Markov models with arbitrary depth structure can easily be developed. In all cases we further address two practical issues: missing observations to unstable tracking and the use of partially labelled data to improve training accuracy. Motivated by real-world problems, our application contribution is a framework to recognize complex activities of daily livings (ADLs) and detect anomalies to provide better intelligent caring services for the elderly.Coarser activities with self duration distributions are represented using the CxHSMM. Complex activities are made of a sequence of coarser activities and represented at the top level in the CxSHSMM. Intensive experiments are conducted to evaluate our solutions against existing methods. In many cases, the superiority of the joint modeling and the Coxian parameterization over traditional methods is confirmed. The robustness of our proposed models is further demonstrated in a series of more challenging experiments, in which the tracking is often lost and activities considerably overlap. Our final contribution is an application of the switching Coxian model to segment education-oriented videos into coherent topical units. Our results again demonstrate such segmentation processes can benefit greatly from the joint modeling of duration and hierarchy
Sensitivity analysis in HMMs with application to likelihood maximization
International audienceThis paper considers a sensitivity analysis in Hidden Markov Models with continuous state and observation spaces. We propose an Infinitesimal Perturbation Analysis (IPA) on the filtering distribution with respect to some parameters of the model. We describe a methodology for using any algorithm that estimates the filtering density, such as Sequential Monte Carlo methods, to design an algorithm that estimates its gradient. The resulting IPA estimator is proven to be asymptotically unbiased, consistent and has computational complexity linear in the number of particles. We consider an application of this analysis to the problem of identifying unknown parameters of the model given a sequence of observations. We derive an IPA estimator for the gradient of the log-likelihood, which may be used in a gradient method for the purpose of likelihood maximization. We illustrate the method with several numerical experiments
Localizing the Latent Structure Canonical Uncertainty: Entropy Profiles for Hidden Markov Models
This report addresses state inference for hidden Markov models. These models
rely on unobserved states, which often have a meaningful interpretation. This
makes it necessary to develop diagnostic tools for quantification of state
uncertainty. The entropy of the state sequence that explains an observed
sequence for a given hidden Markov chain model can be considered as the
canonical measure of state sequence uncertainty. This canonical measure of
state sequence uncertainty is not reflected by the classic multivariate state
profiles computed by the smoothing algorithm, which summarizes the possible
state sequences. Here, we introduce a new type of profiles which have the
following properties: (i) these profiles of conditional entropies are a
decomposition of the canonical measure of state sequence uncertainty along the
sequence and makes it possible to localize this uncertainty, (ii) these
profiles are univariate and thus remain easily interpretable on tree
structures. We show how to extend the smoothing algorithms for hidden Markov
chain and tree models to compute these entropy profiles efficiently.Comment: Submitted to Journal of Machine Learning Research; No RR-7896 (2012
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