1. A computer-implemented method for identifying training documents for a content classifier for a plurality of categories, the training being with respect to a category of the plurality of categories, the method comprising: configuring one or more computer processors to perform an operation comprising: receiving a plurality of documents; identifying, from the received plurality of documents, one or more training documents to represent the category to use in training the content classifier; and training the content classifier using each identified document as a positive training example representing the category.