Frequently, the documents needed by a user are available only via multiple search engines. For example, research papers about a particular subject may be found from the search engines of related digital libraries and journals. It is inconvenient for the user to search these search engines separately. An effective way to address this problem is to employ a metasearch engine, which is a system that provides unified access to multiple existing search systems. When a metasearch engine receives a user query, it passes the query to its underlying search engines. The results returned by the search engines, are then combined by the metasearch engine to form a single ranked list for presentation to the user [2].