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 a 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; ranking the nested structure of clusters of questions according to number of questions in each cluster; associating each cluster of questions with a topic; generating a usage report based on the nested structure of clusters of questions, wherein generating the usage report comprises: identifying one or more topics having a number of questions that is below a predetermined threshold, and recommending removing or replacing documents from the corpus in the one or more clusters corresponding to the one or more topics; presenting the usage report responsive to a requesting user; and subsequent to the user removing or replacing documents from the corpus, thereby creating a modified corpus of documents, utilizing the modified corpus of documents to generate an answer to a question submitted to the question answering system.