40. The system of claim 35 , wherein determining that the resource is a new resource comprises determining that an age of the resource satisfies a threshold.
35. 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: receiving a first search result obtained in response to a query, wherein the first search result identifies a resource and has an associated score S; computing a first probability that a particular search result provided in response to the query in a recent time period will be selected when a particular user is presented with an opportunity to select the particular search result; computing a second probability that the particular search result provided in response to the query in an overall time period will be selected when the particular user is presented with an opportunity to select the particular search result; computing a QtoA ratio of the first probability to the second probability; determining, based on the QtoA ratio, that users in the recent time period preferred newer resources for the query than users in the overall time period; determining that the resource is a new resource; and applying a boosting function to S to compute a new score S′ for the resource, wherein S′ has a boosted value that is higher relative to boosted scores of other resources that are older than the resource.