Methods, systems, and apparatus, including computer program products, for testing web search scoring functions. A query is received. A first and a second scoring function are selected by receiving search results responsive to the query; applying candidate scoring functions to the search results to determine scores for the search results for each candidate scoring function; identifying pairs of the candidate scoring functions, and calculating a diversity score for each of the pairs. A pair of candidate scoring functions is chosen from the one or more pairs of candidate scoring functions based on the diversity scores, and the alpha function is selected as the first scoring function and the beta function is selected as the second scoring function. The plurality of search results are presented in an order according to scores from the first scoring function and are presented in an order according to scores from the second scoring function.