36. A system comprising: one or more computers programmed 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.
39. The system of claim 36 , wherein the factor is further based on how statistically significant the quality of result difference is.