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March 25, 2006

ExpertRank: Authoritative Search

After retiring Jeeves, Ask also retired Teoma, a search engine started in 2000 by Apostolos Gerasoulis and later acquired by Ask Jeeves.

The algorithm behind Teoma was rebranded ExpertRank: "Ask's ExpertRank algorithm provides relevant search results by identifying the most authoritative sites on the Web. With Ask search technology, it's not just about who's biggest: it's about who's best. Our ExpertRank algorithm goes beyond mere link popularity (which ranks pages based on the sheer volume of links pointing to a particular page) to determine popularity among pages considered to be experts on the topic of your search. This is known as subject-specific popularity. Identifying topics (also known as "clusters"), the experts on those topics, and the popularity of millions of pages amongst those experts -- at the exact moment your search query is conducted -- requires many additional calculations that other search engines do not perform. The result is world-class relevance that often offers a unique editorial flavor compared to other search engines."

ExpertRank is an evolution of IBM's CLEVER project, a search engine that never made it to public. "Clever attempts to ensure that the information it retrieves is useful by pointing people toward either of two classes of sites: authorities and hubs. An authority is a site to which many other sites have links, which Dom sees as implied endorsements of the site's usefulness. A hub is a site that has links to many other sites, and is therefore a potentially good reference. Clever's job is to identify the best hubs (those that link to the best authorities) and the best authorities (those that are linked to by the best hubs)."

The difference between PageRank and ExpertRank is that for ExpertRank the quality of the page is important and that quality is not absolute, but it's relative to a subject.

"Clever starts with 200 pages that are the result of an ordinary keyword search. It then adds all pages that link to, or are linked to by, one of those 200 pages. This step typically swells the set of pages to 1,000 or more. Clever initially assigns each page a hub score of one and an authority score of one. It sums up all the authority scores to get a page's hub score, and sums up all the hub scores to get a page's authority score. Then it repeat the process some five times until the system has identified the hubs that link to the top-scoring authorities and the authorities that are linked to by the top-scoring hubs."

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