Computer Science Department
School of Computer Science, Carnegie Mellon University


Using EM to Classify Text from Labeled and Unlabeled Documents

Kamal Nigam, Andrew McCallum, Sebastian Thrun, Tom Mitchell

May 1998

Keywords: Learned text classifiers, algorithmn for learning from labeled and unlabeled text

This paper shows that the accuracy of learned text classifiers can be improved by augmenting a small number of labeled training documents with a large pool of unlabeled documents. This is significant because in many important text classification problems obtaining classification labels is expensive, while large quantities of unlabeled documents are readily available. We present a theoretical argument showing that, under common assumptions, unlabeled data contain information about the target function. We then introduce an algorithm for learning from labeled and unlabeled text, based on the combination of Expectation-Maximization with a naive Bayes classifier. The algorithm first trains a classifier using the available labeled documents, and probabilistically labels the unlabeled documents. It then trains a new classifier using the labels for all the documents, and iterates. Experimental results, obtained using text from three different real-world tasks, show that the use of unlabeled data reduces classification error by up to 30%.

20 pages

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