1. A method performed by one or more data processing apparatus, the method comprising: receiving a first query associated with a user request; determining a score for a word in the first query based at least in part on a first count of activities representing user interaction with content items served for display in response to a past query that included the word and a second count of activities representing user interaction with content items served for display in response to another past query that did not include the word; selecting a keyword derived from the first query based at least in part on the score; and identifying candidate content items using the selected keyword.
1. A method performed by one or more data processing apparatus, the method comprising: receiving a first query associated with a user request; determining a score for a word in the first query based at least in part on a first count of activities representing user interaction with content items served for display in response to a past query that included the word and a second count of activities representing user interaction with content items served for display in response to another past query that did not include the word; selecting a keyword derived from the first query based at least in part on the score; and identifying candidate content items using the selected keyword. 4. The method of claim 1 , wherein one of the candidate content items is an advertisement.