23. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform actions comprising: receiving a plurality of queries sequentially, each query being received from a respective user; and performing the following operations with respect to each query and respective user: selecting a pair of candidate scoring functions from a group of at least three candidate scoring functions, the pair including a first candidate scoring function and a second candidate scoring function, the selecting being based at least in part on a ranking of the candidate scoring functions in the group; presenting search results, which are responsive to the query, ordered according to scores from the first candidate scoring function, to the respective user; presenting the search results, which are responsive to the query, ordered according to scores from the second candidate scoring function, to the respective user; receiving user input, from the respective user, selecting one of the first and second candidate scoring functions over the other; and using the user input to update the ranking of the candidate scoring functions in the group.
24. The system of claim 23 , wherein using the user input to update the ranking of the candidate scoring functions in the group comprises: adding the user input to a set of multiple pair-wise user comparisons of scoring functions; and generating the ranking of the candidate scoring functions in the group based at least in part on the set of multiple pair-wise user comparisons.