1. A computer-implemented method comprising: receiving a query at a computer; selecting a first candidate scoring function and a second candidate scoring function from a group of at least three candidate scoring functions; presenting search results, which are responsive to the query, ordered according to scores from the first candidate scoring function; presenting the search results, which are responsive to the query, ordered according to scores from the second candidate scoring function; receiving user input selecting one of the first and second candidate scoring functions over the other; and using the user input to inform future selections of pairs of candidate scoring functions from the group of candidate scoring functions.