1. A method, in a question answering system, for generating a usage report based on question categorization, the method comprising: clustering documents from a corpus of documents to form a nested structure of clusters of documents; recording questions answered by question answering system in a database in association with answers returned and answer confidence values; mapping the questions to the nested structure of clusters of documents to form a nested structure of clusters of questions; generating a usage report based on the nested structure of clusters of questions; and presenting the usage report responsive to a requesting user.