18. The system of claim 17 , wherein the operations further comprise: determining that a no-match ratio between the no-match score and the query count does not satisfy a no-match ratio threshold; and in response to determining that the no-match ratio does not satisfy the no-match ratio threshold, removing the query term substitution rule.
17. 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 operations comprising: determining a no-match score, a click score, and a query count for a query term substitution rule that is used to revise queries that include a query term, to include a substitute term for the query term, wherein the no-match score for the query term substitution rule indicates a quantity of revised search queries that (i) were generated using the query term substitution rule to include the query term and the substitute term, and (ii) were each used in identifying a respective set of search results that (I) were associated with text that did not include the substitute term, and (II) were included on a respective initial search engine results page, wherein the click score for the query term substitution rule indicates a quantity of revised search queries that (i) were generated using the query term substitution rule to include the query term and the substitute term, and (ii) were each used in identifying a respective set of search results, one or more of which were (I) associated with text that included the substitute term, (II) were selected by a user, and (III) were included on a respective initial search engine results page, wherein the query count for the query term substitution rule indicates a quantity of revised search queries that were generated using the query term substitution rule to include the query term and the substitute term; and determining whether to remove the query term substitution rule from a query revision model based on the no-match score, the click score, and the query count.