Google's Pagerank and Beyond: The Science of Search Engine Rankings

By Amy N. Langville; Carl D. Meyer | Go to book overview

Chapter Eleven
The HITS Method for Ranking Webpages

If you’re a sports fan, you’ve seen those “—— is Life” t-shirts, where the blank is filled in by a sport like football, soccer, cheerleading, fishing, etc. After reading the first ten chapters of this book, you might be ready to declare “Google is Life.” But your mom probably told you long ago that “there’s more to life than sports.” And there’s more to search than Google. In fact, there’s Teoma, and Alexa, and A9, to name a few. The next few chapters are devoted to search beyond Google. This chapter focuses specifically on one algorithm, HITS, the algorithm that forms the basis of Teoma’s popularity ranking.


11.1 THE HITS ALGORITHM

We first introduced HITS, the other system for ranking webpages by popularity back in Chapter 3. Since that was many pages ago, we review the major points regarding HITS. HITS, which is an acronym for Hypertext Induced Topic Search, was invented by Jon Kleinberg in 1998—around the same time that Brin and Page were working on their PageRank algorithm. HITS, like PageRank, uses the Web’s hyperlink structure to create popularity scores associated with webpages. However, HITS has some important differences. Whereas the PageRank method produces one popularity score for each page, HITS produces two. Whereas PageRank is query-independent, HITS is query-dependent. HITS thinks of webpages as authorities and hubs. An authority is a page with many inlinks, and a hub is a page with many outlinks. Authorities and hubs deserve the adjective good when the following circular statement holds: Good authorities are pointed to by good hubs and good hubs point to good authorities. And so every page is some measure of an authority and some measure of a hub. The authority and hub measures of HITS have been incorporated into the CLEVER project at IBM Almaden Research Center [2]. HITS is also part of the ranking technology used by the new search engine Teoma [150].

After this recap, we are ready to translate these words about what HITS does into mathematics. Every page i has both an authority scorexi and a hub scoreyi. Let E be the set of all directed edges in the web graph and let eij represent the directed edge from node i to node j. Given that each page has somehow been assigned an initial authority score

and hub score , HITS successively refines these scores by computing

These equations, which were Kleinberg’s original equations, can be written in matrix

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