11. A system of one or more computers configured to perform operations comprising: obtaining user feedback associated with quality of a document including receiving user selections of the document and a plurality of other documents presented by a document search service; adjusting a measure of relevance for the document, including: determining a first click trend for the document, the first click trend comprising a plurality of first date intervals and, for each of the first date intervals, a respective volume of user selections of the document in search results provided by the document search service within the date interval; determining a second click trend for the other documents, the second click trend comprising a plurality of second date intervals and, for each of the second date intervals, a respective volume of user selections of the other documents in search results provided by the document search service within the date interval, where contributions of the user selections of the document and the user selections of the other documents to the first click trend and the second click trend, respectively, are based on temporal distance between a current time and times of the user selections; comparing the first click trend to the second click trend and determining that a first rate of change for the first click trend is larger than a second rate of change for the second click trend; and as a consequence of determining that the first rate of change is larger than the second rate of change, increasing the measure of relevance for the document; and outputting the measure of relevance to a ranking engine for ranking of search results, for a search that identifies the document as a search result, the ranking of search results including ranking the document using the measure of relevance.
11. A system of one or more computers configured to perform operations comprising: obtaining user feedback associated with quality of a document including receiving user selections of the document and a plurality of other documents presented by a document search service; adjusting a measure of relevance for the document, including: determining a first click trend for the document, the first click trend comprising a plurality of first date intervals and, for each of the first date intervals, a respective volume of user selections of the document in search results provided by the document search service within the date interval; determining a second click trend for the other documents, the second click trend comprising a plurality of second date intervals and, for each of the second date intervals, a respective volume of user selections of the other documents in search results provided by the document search service within the date interval, where contributions of the user selections of the document and the user selections of the other documents to the first click trend and the second click trend, respectively, are based on temporal distance between a current time and times of the user selections; comparing the first click trend to the second click trend and determining that a first rate of change for the first click trend is larger than a second rate of change for the second click trend; and as a consequence of determining that the first rate of change is larger than the second rate of change, increasing the measure of relevance for the document; and outputting the measure of relevance to a ranking engine for ranking of search results, for a search that identifies the document as a search result, the ranking of search results including ranking the document using the measure of relevance. 12. The system of claim 11 , the operations further comprising determining the measure of relevance for the document within a context of a search query for which the document is returned by the document search service.