7. A computer-readable storage medium containing instructions for controlling a computer system to calculate relevance of a document to a user, by a method comprising: providing click-through data generated when users submitted queries to a search engine and selected a document from results provided by the search engine; identifying user, query, and document triplets from the click-through data, each triplet indicating that the user of the triplet submitted the query of the triplet and the user selected the document of the triplet from results of the query provided by the search engine; receiving from a user a query; searching for documents to be provided as results of the received query; for each document of the results of the received query, determining a probability that the user from whom the query was received will find the document relevant by performing a smoothing of the identified triplets to account for sparseness of the triplets and calculating the probability based on the smoothed triplets, wherein the determined probability is based at least in part on identified triplets identified for users other than the user from whom the query was received and wherein the probability is the probability of the document given the user and the query when the corresponding user, query, and document triplet is identified and is the probability of the document given the query otherwise and wherein a back-off factor is applied to the probability; and displaying an indication of the documents to the user from whom the query was received in an order based on the determined probabilities of the documents.
12. The computer-readable storage medium of claim 7 wherein the smoothing uses a combination of back-off, clustering, and content similarity techniques.