2001 ConditionalRandomFields

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Subject Headings: Conditional Random Fields, Linear-Chain Conditional Random Field, Named Entity Recognition, Global Collective Classification Algorithm.


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We present conditional random fields, a framework for building probabilistic models to segment and label sequence data. Conditional random fields offer several advantages over hidden Markov models and stochastic grammars for such tasks, including the ability to relax strong independence assumptions made in those models. Conditional random fields also avoid a fundamental limitation of maximum entropy Markov models (MEMMs) and other discriminative Markov models based on directed graphical models, which can be biased towards states with few successor states. We present iterative parameter estimation algorithms for conditional random fields and compare the performance of the resulting models to HMMs and MEMMs on synthetic and natural-language data.

1. Introduction

The need to segment and label sequences arises in many different problems in several scientific fields. Hidden Markov models (HMMs) and stochastic grammars are well understood and widely used probabilistic models for such problems. In computational biology, HMMs and stochastic grammars have been successfully used to align biological sequences, find sequences homologous to a known evolutionary family, and analyze RNA secondary structure (Durbin et al., 1998). In computational linguistics and computer science, HMMs and stochastic grammars have been applied to a wide variety of problems in text and speech processing, including topic segmentation, part-of-speech (POS) tagging, information extraction, and syntactic disambiguation (Manning & Schütze, 1999).

3. Conditional Random Fields

In what follows, X is a random variable over data sequences to be labeled, and Y is a random variable over corresponding label sequences. All components [math]Y_i[/math] of [math]Y[/math] are assumed to range over a finite label alphabet A. For example, [math]X[/math] might range over natural language sentences and [math]Y[/math] range over part-of-speech taggings of those sentences, with A the set of possible part-of-speech tags. The random variables [math]X[/math] and [math]Y[/math] are jointly distributed, but in a discriminative framework we construct a conditional model [math]p(Y\vert X)[/math] from paired observation and label sequences, and do not explicitly model the marginal [math]p(X)[/math].

Definition: Let [math]G = (V,E)[/math] be a graph such that [math]Y = (Y_v)_{v \in V}[/math], so that [math]Y[/math] is indexed by the vertices of [math]G[/math]. Then [math](X,Y)[/math] is a conditional random field in case, when conditioned on [math]X[/math], the random variables [math]Y_v[/math] obey the Markov property with respect to the graph: [math]p(Y_v \vert X,Y_w,w \neq v) = p(Y_v \vert X,Y_w, \lt math\gt w[/math] \sim v)</math>, where w~v means that [math]w[/math] and [math]v[/math] are neighbors in [math]G[/math].


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 AuthorvolumeDate ValuetitletypejournaltitleUrldoinoteyear
2001 ConditionalRandomFieldsJohn D. Lafferty
Andrew McCallum
Fernando Pereira
Conditional Random Fields: Probabilistic Models for Segmenting and Labeling Sequence DataICML 2001http://www.cis.upenn.edu/~pereira/papers/crf.pdf2001