Inverse Document Frequency (IDF): A Measure of Deviations from Poisson

Abstract

Low frequency words tend to be rich in content, and vice versa. But not all equally frequent words are equally mean!ngful. We will use inverse document frequency (IDF), a quantity borrowed from Information Retrieval, to distinguish words like somewhat and boycott. Both somewhat and boycott appeared approximately 1000 times in a corpus of 1989 Associated Press articles, but boycott is a better keyword because its IDF is farther from what would be expected by chance (Poisson).