21. A non-transitory machine-readable storage device having instructions stored thereon which, when executed by at least one data processing apparatus, cause the at least one data processing apparatus to perform operations comprising: determining historical click-through rates over a plurality of time periods for a first search result responsive to a query and for a second search result responsive to the query wherein the first and second search results refer to different respective web pages; calculating click-fractions for one or more of the plurality of time periods based on the determined historical click-through rates of the first and second search results; determining that a particular click-fraction of the calculated click-fractions in a first time period of the plurality of time periods exceeds a minimum change threshold; receiving the query from a user during a second time period that chronologically follows the plurality of time periods; obtaining search results responsive to the query; adjusting a ranking of the first search result in the obtained search results during the second time period; and providing the search results including the adjusted ranking of the first search result to the user.
21. A non-transitory machine-readable storage device having instructions stored thereon which, when executed by at least one data processing apparatus, cause the at least one data processing apparatus to perform operations comprising: determining historical click-through rates over a plurality of time periods for a first search result responsive to a query and for a second search result responsive to the query wherein the first and second search results refer to different respective web pages; calculating click-fractions for one or more of the plurality of time periods based on the determined historical click-through rates of the first and second search results; determining that a particular click-fraction of the calculated click-fractions in a first time period of the plurality of time periods exceeds a minimum change threshold; receiving the query from a user during a second time period that chronologically follows the plurality of time periods; obtaining search results responsive to the query; adjusting a ranking of the first search result in the obtained search results during the second time period; and providing the search results including the adjusted ranking of the first search result to the user. 26. The storage device of claim 21 , wherein a click-fraction of the calculated click fractions is a ratio of the determined historical click-through rate of the first search result in the time period to the determined historical click-through rate of the second search result in the time period.