36. The system of claim 27 , wherein, with respect to each of the association rules, using the association rule in categorizing the documents of the set with regard to the document category includes: identifying a set of negated words, wherein the set of negated words includes words negated by negation terms in the association rule, and wherein the set of negated words is empty when the association rule does not include any negation terms; identifying a set of distinct words, wherein the set of distinct words includes words or expressions used in distinct terms of the association rule; categorizing documents of the set as being included in the document category when both of the following conditions are satisfied: each word or expression in the set of distinct words is included in the document; and each negated word in the set of negated words is absent from the document.
27. A system, comprising: one or more data processors; and a non-transitory computer readable storage medium containing instructions which when executed on the one or more data processors, cause the one or more data processors to perform operations including: accessing a representation of a document category; accessing a set of documents, wherein each of the documents of the set is associated with a label indicating whether or not the document is associated with the document category; compiling a list of terms, wherein the list of terms includes distinct terms found in the documents of the set; computing information gain of each of the terms, wherein information gain is computed with regard to the document category; sorting the list by ordering the terms based on respective information gain; creating multiple association rules, wherein at least one of the association rules includes a logical conjunction of at least two terms of the list and one or more of the association rules includes at least one of the distinct terms; and performing an association rule analysis that includes performing the following operations with respect to each of the association rules: obtaining categorization results by using the association rule in categorizing each of the documents of the set with regard to the document category; recording the categorization results; and evaluating a precision of the association rule based on the respective categorization results.