10. A system comprising: a storage device having instructions stored thereon; and data processing apparatus operable to execute the instructions 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.
15. The system of claim 10 wherein the respective counts for the first and second corpora are derived from language-specific data or country-specific data, and wherein determining a relevance of a corpus for the search query further comprises selecting which of the data to use for the respective counts based on a determination that the selected data will provide a reliable statistic.