1. In a computerized search system in which queries are submitted by users who receive, in response, a list of documents selected from a corpus of documents wherein the list comprises documents deemed responsive to a user's query, a method of determining relevance of the documents comprising: obtaining, at a server computer comprising a processor, the query from a user; determining initial probabilities that a plurality of categories of a taxonomy contain documents relevant to the query, at least one of the initial probabilities being non-zero, wherein the initial probabilities are numeric values; determining a relevance of the documents matching the query in each category having non-zero initial probability; and determining a relevance of documents to the query based on the initial probabilities of the plurality of categories and the relevance of the documents matching the query, wherein determining the relevance of documents to the query comprises: for each particular category containing a particular document, determining a value of the relevance of a particular document to the query by multiplying a determined relevance of the particular document matching the query by the initial probability that the particular category includes relevant documents; generating updated probabilities that the categories of the taxonomy contain relevant documents by multiplying individual ones of the relevance of documents to the query by a weighting factor, wherein the weighting factors used to generate the updated probabilities decay monotonically with the relevance value of the documents to the query; determining an updated relevance of documents to the query based on the updated probabilities and the relevance of the documents matching the query; and summing the updated relevance values to determine the relevance of the documents to the query.