1. A method performed by data processing apparatus, the method comprising: obtaining search query data, the search query data including a plurality of search queries submitted by users, each search query being associated with one or more responsive search results and user selection data for the one or more search results, wherein the user selection data identifies which search results were selected by users; obtaining a plurality of score improvement lists, each score improvement list being associated with a point value, each score improvement list being an ordered list of adjusters, wherein the adjusters are serially applied in order of the adjusters in the ordered list to initial scores of a group of search results to determine a final scoring of the search results; iteratively selecting pairs of score improvement lists from a pool of score improvement lists, the pool of score improvement lists including the plurality of score improvement lists; automatically for each selected pair: separately applying the ordered list of adjusters for each score improvement list included in the selected pair to one or more search results associated with a search query from the search query data, in response to the applying, identifying, for each score improvement list in the selected pair, respective ordered search results responsive to the search query, determining a winning and losing score improvement list in the selected pair from the respective ordered search results and the user selection data associated with the search query, increasing the point value associated with the winning score improvement list, and decreasing the point value associated with the losing score improvement list; repeating testing for respective pairs of score improvement lists until ending criteria are reached; and selecting one or more score improvement lists based on the respective point values after the repeated testing.
5. The method of claim 1 , wherein the ending criteria include one or more of the following: the point values associated with the pool of score improvement lists stabilize, a number of iterations of repeated testing of score improvement lists reaches an iteration threshold, or a point value of the point values associated with the pool of score improvement lists reaches a threshold amount more than the other point values associated with the pool of score improvement lists.