16. The system of claim 15 , wherein the operations comprise: receiving a second query; selecting a third candidate scoring function and a fourth candidate scoring function from a top number of scoring functions in the ranking for the candidate scoring functions in the group; presenting search results, which are responsive to the second query, ordered according to scores from the third candidate scoring function; presenting the search results, which are responsive to the second query, ordered according to scores from the fourth candidate scoring function; receiving second user input selecting one of the third and fourth candidate scoring functions over the other; and updating the ranking for the candidate scoring functions in the group based at least in part on the second user input.
16. The system of claim 15 , wherein the operations comprise: receiving a second query; selecting a third candidate scoring function and a fourth candidate scoring function from a top number of scoring functions in the ranking for the candidate scoring functions in the group; presenting search results, which are responsive to the second query, ordered according to scores from the third candidate scoring function; presenting the search results, which are responsive to the second query, ordered according to scores from the fourth candidate scoring function; receiving second user input selecting one of the third and fourth candidate scoring functions over the other; and updating the ranking for the candidate scoring functions in the group based at least in part on the second user input. 17. The system of claim 16 , wherein the operations comprise: applying each respective candidate scoring function of the group to the search results to determine respective scores for the search results for the respective candidate scoring function; and comparing results located in same positions in respective sets of the search results, ordered according to respective candidate scoring functions, for each of the pairs of candidate scoring functions; wherein the selecting is based on output of the comparing.