28. A computer-implemented method 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.
28. A computer-implemented method 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. 33. The method of claim 28 , further comprising determining that a first past version of the first document during a first time period is different from a second past version of the first document during a second time period before modifying the first quality of result statistic.