1. A computer-implemented method for determining the relevance of a particular document to a particular query, the method comprising: based on session information, identifying a plurality of query sessions for the particular query; wherein, for each session in the plurality of query sessions, the session information comprises data indicating an ordered list of documents, wherein the ordered list of documents was presented, in order, as a search results for the session; for each session in the plurality of query sessions: identifying click-through information associated with the session, wherein the click-through information comprises data indicating which one or more documents were selected by a user associated with the session from the ordered list of documents presented during the session; estimating a score for the particular document relative to the particular query; wherein the score for the particular document is based on, for each session of the plurality of query sessions in which the particular document was presented as a search result: click-through information indicating whether the particular document was selected in the session, and click-through information, for the same session, corresponding to other documents in the ordered list of documents that was presented during the session; wherein the method is performed by one or more computing devices.