1. 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, the one or more time trend statistics estimating changes in the quality of result statistics over time, wherein each of the one or more time trend statistics comprises a quality of result difference between a first quality of result statistic for the first document as a search result for the first query during a first time period and a second quality of result statistic for the first document as a search result for the first query during a second time period; 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.