9. A non-transitory computer storage medium encoded with instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: obtaining first search results that were generated for a search query; determining, by operation of a system, a first search results score associated with the first search results, wherein the first search results score is based on (i) a respective rank of each first search result, and (ii) a respective first popularity score associated with each first search result; revising the search query using a query revision rule; obtaining second search results that were generated for the revised search query; determining a second search results score associated with the second search results, wherein the second search results score is based on (i) a respective rank of each second search result, and (ii) a respective second popularity score associated with each second search result; determining that a difference between the first search results score and the second search results score satisfies a specified threshold; and in response to determining that a difference between the first search results score and the second search results score satisfies the specified threshold, identifying the query revision rule as a good revision rule for the search query.
15. The non-transitory computer storage medium of claim 9 , wherein determining a second search results score associated with the second search results, wherein the second search results score is based on (i) respective ranks of search results in the second search results, and (ii) respective popularity scores associated with the search results in the second search results comprises: determining, for each search result in the second search results, a respective position score, the respective position score being a product of a rank of the search result and a popularity score associated with the search result; and determining the second search results score based on a sum of the respective position scores for the second search results.