48. The non-transitory computer readable storage medium of claim 40 , wherein the one or more programs include instructions for evaluating the identified predicted performance function with respect to a particular search result at a plurality of potential search result positions to produce values, corresponding to predicted user click through rates, for each position in the plurality of potential search result positions.
40. A non-transitory computer readable storage medium for storing one or more programs to be executed by one or more processors of a computer system, the one or more programs comprising: instructions for determining a correlation, for each search context of a plurality of search contexts, for each scoring primitive of a plurality of scoring primitives, and for a set of previously executed search queries that are consistent with the search context, between the scoring primitive and actual user selections of results of the previously executed search queries by a plurality of users; instructions for performing machine learning, for each search context, on the correlations to identify a predicted performance function comprising a weighted subset of the scoring primitives that meet predefined predictive quality criteria, wherein the identified predicted performance function is associated with the search context; and instructions for receiving and executing a user submitted search query, submitted by a user, to produce a set of search results, including associating the user submitted search query with a respective search context of the plurality of search contexts, and ordering at least a portion of the search results in accordance with the identified predicted performance function for the search context associated with the user submitted search query.