44. A computer storage device having instructions stored thereon that, when executed by data processing apparatus, cause the data processing apparatus to perform operations comprising: calculating one or more time trend statistics for a plurality of quality of result statistics for a first document as a search result for a first query, each of the quality of result statistics corresponding to a different time period and being an estimate of a respective portion of users that found the first document relevant to the first query out of a total number of users who viewed the first document as a search result for the first query during the time period, the one or more time trend statistics estimating changes in the quality of result statistics over time; generating a first modified quality of result statistic by modifying a first quality of result statistic for the first document as a search result for the first query by a factor, where the factor is based on the one or more time trend statistics; and providing the first modified quality of result statistic as an input to a document ranking process for the first document and the first query.
48. The storage device of claim 44 wherein the operations further comprise: generating a second modified quality of result statistic for the first document and a second different query according to the one or more time trend statistics; and providing the second modified quality of result statistic as an input to the document ranking process for the first document and the second query.