4. The method of claim 1 , further comprising: generating updated probabilities that the leaf categories of the taxonomy contain relevant documents, the updated probabilities being based on the relevance of documents to the query; and determining an updated relevance of documents to the query based on the updated probabilities that the leaf categories of the taxonomy contain relevant documents and the relevance of the documents matching the query.
8. The method of claim 4 , further comprising repeatedly generating the updated probabilities that the leaf categories of the taxonomy contain relevant documents based on the previous updated probabilities that the leaf categories of the taxonomy contain relevant documents and re-determining the updated relevance of documents to the query.