1. A method implemented by data processing apparatus, the method comprising: determining, for a plurality of search results responsive to a query, a respective count of times search results in the plurality of search results that refer to documents in a base corpus have been presented, and a respective count of times search results in the plurality of search results that refer to documents in the base corpus have been selected, wherein the respective counts for presentations and selections of search results that refer to documents in the base corpus are for searches initiated by users in a plurality of different countries who employ a specific language; determining, for the plurality of search results responsive to the query, a respective count of times search results in the plurality of search results that refer to documents in a second corpus have been presented, and a respective count of times search results in the plurality of search results that refer to documents in the second corpus have been selected, wherein the respective counts for presentations and selections of search results that refer to documents in the second corpus are for searches initiated by users in the plurality of different countries who employ the specific language; calculating a click through rate of the base corpus for the query based at least in part on the respective counts for the base corpus; calculating a click through rate of the second corpus for the query based at least in part on the respective counts for the second corpus; calculating a measure of relative relevance based at least in part on a ratio of the second corpus click through rate to the base corpus click through rate; and providing the measure of relative relevance to a ranking engine for ranking of search results for a search corresponding to the query; and wherein fewer search results of the plurality refer to documents in the second corpus than to documents in the base corpus.