Google's algorithms rank web pages by analysing their hyperlinks in a series of iterative cycles. Whereas most search engines only associate the text of a link with the page the link is on, Google also associates it with the page the link points to. This allows it to cover many more pages than it actually crawls, even yielding links to sites that bar search engines' crawler programs. Clever, a prototype search engine being developed at IBM Almaden, takes the citation analogy further. Like Google, it produces a ranking of authorities, but it also generates a list of 'hubs', pages that have many links to authorities.
Hubs are akin to review articles, which cite many top papers.
Those that link to many of the most highly cited authorities are given higher rankings than those that link to less well-respected sites. Users get not only the top hits, but the hubs provide a good starting point for browsing. However, search strategies that rely on analysing hyperlinks are no panacea. New pages will have few links to them, and may be missed. And such strategies may be of little use to a scientist seeking highly specific information found on specialist sites with few incoming links.