1. A method for operating a probabilistic document categorizer having an associated vocabulary of words and an associated plurality of probabilistic categorizer parameters derived from a collection of documents, the method comprising: receiving a new document; and updating the probabilistic categorizer parameters to reflect addition of the new document to the collection of documents based on vocabulary words contained in the new document, a category of the new document, and a collection size parameter indicative of an effective total number of instances of vocabulary words in the collection of documents.