Another challenge faced in the implementation of federated search engines is scalability. It is difficult to maintain the performance, the response speed, of a federated search engine as it combines more and more information sources together. One implementation of federated search that has begun to address this issue is WorldWideScience, hosted by the U.S. Department of Energy's Office of Scientific and Technical Information. WorldWideScience is composed of more than 40 information sources, several of which are federated search portals themselves. One such portal is Science.gov which itself federates more than 30 information sources representing most of the R&D output of the U.S. Federal government. Science.gov returns its highest ranked results to WorldWideScience, which then merges and ranks these results with the search returned by the other information sources that comprise WorldWideScience. This approach of cascaded federated search enables large number of information sources to be searched via a single query.