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 indicated by 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 one or more documents selected by a user associated with the session; based on the click-through information associated with the session: performing a first determination of whether or not, during the session, the user selected the particular document; performing a second determination of whether or not, during the session, the user selected other documents in the ordered list of documents for the session; based on a set of the first determinations and a set of the second determinations, estimating a score for the particular document, said score representing a probability that a given user will find the particular document relevant to the particular query.