1. A method implemented by at least one data processing apparatus, the method 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; obtaining search results responsive to the query during a second time period that chronologically follows the plurality of time periods; adjusting a ranking of the first search result in the obtained search results during the second time period; and wherein determining, calculating, obtaining and adjusting are performed by the at least one data processing apparatus.
4. The method of claim 1 wherein the determined historical click-through rates are for a first geographic region.