Generalized Linear Classifiers in NLP

For the better part of a decade machine learning methods like maximum entropy and support vector machines have been a major part of many NLP applications such as parsing, semantic role labeling, ontology induction, machine translation, and summarization. Many of these models fall into the class of Generalized Linear Classifiers, which are characterized by defining a prediction boundary as a linear combination of input features and their weights. In this course we will cover many of the important aspects of generalized linear classifiers including: training methods, min error vs. max likelihood, distribution free methods, online vs. batch, generative vs. discriminative, structured models, distributed algorithms, and extensions beyond linear predictors through kernels. The course assumes familiarity with basic concepts from statistics, calculus and linear algebra.

Date: October 19th, 2009; Lecturer: Ryan McDonald

Outline (subject to change)
 10-12 Introduction, feature representations, loss functions, perceptron, margin, SVMs, logistic regression (Max Ent), stochastic gradient descent 13-15 Parallelization, structured learning including conditional random fields, Kernels 15-17 Left-overs, practical

Practical

An implementation of the perceptron algorithm and some extensions handout. Starter code available here, data sets included.

Project suggestions

• Build a structured perceptron algorithm for entity or part-of-speech tagging
• Download and test various linear classifiers for standard NLP problems (MALLET (log. reg. (max. ent.)), libsvm or svm light)
• Download and compare some structured learning algorithms (MALLET (CRF), StructLearn (Perceptron, MIRA), StructSVM)
• Create a kernalized version of the perceptron algorithm
• Implmenet a stochastic gradient ascent/descent algorithm and compare with perceptron
• Implement a parallel logistic regression (simulation)
• Investigate parallelization by weight averaging, when does it work and when does it not work?
• Parsing: show how one can use perceptron and/or CRFs for structured learning of context-free parsing through CKY and inside-outside algorithms
• Prove the equivalence of logistic regression and maximum entropy, i.e., write out both objective functions and show that they are maximized with precisely the same parameters
• Email me about possible data sets (see my webpage for an address).

Literature and resources

• Previous course from 2007 html
• Slides from ESSLLI lecture pdf
• A tutorial by John Blitzer pdf
• SVM tutorials html
• Tutorials by Dan Klein html. Bottom of the page. Check out "Introduction to Classification", "Max Margin Methods for NLP", and "Maxent Models ..."
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