1. A method in a computer system for calculating importance of a document, the method comprising: providing user, query, and document triplets indicating that the user submitted the query and that the user selected the document from a result of the query; receiving an input user, an input query, and an input document as an input triplet, the input query having been submitted by the input user and the input document being a document of a search result for the input query; and determining a probability that the user will find the input document important by performing a smoothing of the provided triplets to account for triplets not provided and calculating the probability based on the smoothing wherein the determined probability is based at least in part on the provided triplets for users other than the input user and wherein the probability is the probability of the input document given the input user and the input query when the corresponding user, query, and document triplet is provided and is the probability of the input document given the input query otherwise and wherein a back-off factor is applied to the probability.
5. The method of claim 1 wherein the probability is based on similarity between an input document and document clusters identified based on relationships between users and queries.