Detecting patterns is a central part of Natural Language Processing. Words ending in -ed tend to be past tense verbs (Chapter 5). Frequent use of will is indicative of news text (Chapter 3). These observable patterns—word structure and word frequency—happen to correlate with particular aspects of meaning, such as tense and topic. But how did we know where to start looking, which aspects of form to associate with which aspects of meaning?
The goal of this chapter is to answer the following questions:
How can we identify particular features of language data that are salient for classifying it?
How can we construct models of language that can be used to perform language processing tasks automatically?
What can we learn about language from these models?
Along the way we will study some important machine learning techniques, including decision trees, naive Bayes classifiers, and maximum entropy classifiers. We will gloss over the mathematical and statistical underpinnings of these techniques, focusing instead on how and when to use them (see Further Reading for more technical background). Before looking at these methods, we first need to appreciate the broad scope of this topic.
Classification is the task of choosing the correct class label for a given input. In basic classification tasks, each input is considered in isolation from all other inputs, and the set of labels is defined in advance. Some examples of classification tasks ...