19. A storage device having instructions stored thereon that, when executed by data processing apparatus, cause the data processing apparatus to perform operations comprising: determining a first measure of a first corpus for a search query as a count of times the search query was submitted to search the first corpus in view of a count of times other queries were submitted to search the first corpus; determining a second measure of a second corpus for the search query as a count of times the search query was submitted to search the second corpus in view of a count of times other queries were submitted to search the second corpus; determining a click measure based on a count of selections of first corpus search results for the search query relative to a count of selections of second corpus search results for the search query; determining a measure of relative relevance of the first corpus and the second corpus for the search query based on the first measure, the second measure and the click measure; and providing the measure of relative relevance to a ranking engine for ranking of search results for a search corresponding to the search query.
19. A storage device having instructions stored thereon that, when executed by data processing apparatus, cause the data processing apparatus to perform operations comprising: determining a first measure of a first corpus for a search query as a count of times the search query was submitted to search the first corpus in view of a count of times other queries were submitted to search the first corpus; determining a second measure of a second corpus for the search query as a count of times the search query was submitted to search the second corpus in view of a count of times other queries were submitted to search the second corpus; determining a click measure based on a count of selections of first corpus search results for the search query relative to a count of selections of second corpus search results for the search query; determining a measure of relative relevance of the first corpus and the second corpus for the search query based on the first measure, the second measure and the click measure; and providing the measure of relative relevance to a ranking engine for ranking of search results for a search corresponding to the search query. 26. The storage device of claim 19 wherein the first corpus comprises web pages and wherein the second corpus comprises images.