1. A computer-implemented method, comprising: determining a topic divergence value, the topic divergence value substantially proportional to a ratio of a first topic word distribution in a topic document corpus to a second topic word distribution in a document corpus, wherein the topic document corpus is a corpus of topic documents related to a topic, and the document corpus is a corpus of documents that includes the topic documents and other documents; determining a candidate topic word divergence value for a candidate topic word, the candidate topic word divergence value substantially proportional to a ratio of a first distribution of the candidate topic word in the topic document corpus to a second distribution of the candidate topic word in the document corpus, wherein the candidate topic word is not a topic word in a topic dictionary for the topic; and determining whether the candidate topic word is a new topic word for the topic based on the candidate topic word divergence value and the topic divergence value.