Opinion mining or sentiment analysis is a hot new research field dedicated to the automatic evaluation of opinions as expressed on social media, product review websites, or other forums. Often, we want to know whether an opinion is positive, neutral, or negative. This is, of course, a form of classification, as seen in the previous section. As such, we can apply any number of classification algorithms. Another approach is to semi-automatically (with some manual editing) compose a list of words with an associated numerical sentiment score (the word "good" can have a score of 5 and the word "bad" a score of -5). If we have such a list, we can look up all the words in a text document and, for example, sum up all the found sentiment scores. The number of classes can be more than three, as in a five-star rating scheme.
We will apply naive Bayes classification to the NLTK movie reviews corpus with the goal of classifying movie reviews as either positive or negative. First, we...