5. The method of claim 1 , further comprising: receiving from each of the plurality of search engines, a ranked list of search results based on the search query, wherein each ranked list includes at least two search results with actual relevance values supplied by the search engine that provided the rank list; and providing an estimated relevance value for each search result without an actual relevance value supplied by the search engine, wherein the estimated relevance value of a particular search result from a particular ranked list is based on a rank of the particular search result in the particular ranked list and actual relevance values and ranks of at least two other search results from the particular ranked list.
7. The method of claim 5 wherein providing the estimated relevance value for each search result without an actual relevance value supplied by the search engine that provided the search results comprises: determining an interpolation function, to represent relevance as a function of rank, for the actual relevance values and the ranks of the at least two others of the search results for the search engine; and evaluating the interpolation function at the rank of the particular search result to estimate the relevance value.