Models for Classifying Documents

Some embodiments provide a method for defining a content relevance model for determining whether a content segment is relevant to a particular category. The method receives a first set of content segments that contain content relevant to the particular category and a second set of content segments that contain content not relevant to the particular category. The method identifies a set of key word sets more likely to appear in the first set of content segments than the second set of content segments. The method defines a content relevance model that comprises a set of groups of word sets and a score for each group, each of the groups of word sets comprising a key word set from the set of key word sets and at least one word set found in a context of the key word set in at least one of the received content segments.