Chapter 6: All Links Are Not Created Equal: 10 Illustrations of Search Engines' Valuation of Links

Editor's Note: It is surprising how little—and how much—has changed in how search engines evaluate and utilize links since this post was written three years ago. While links are no longer the only signals that search engines use to determine the importance and popularity of a web page, they are still the strongest ones. For the most part, the principals of link valuation as explained here still hold water as well, especially if each one is updated within the current context of relevance and quality. As search engines have become more sophisticated, they have become increasingly adept at determining the relevance and quality of links, and they have adjusted the weight placed on those factors when valuing links accordingly.

In 1997, Google's founders created an algorithmic method to determine importance and popularity based on several key principles:

• Links on the web can be interpreted as votes that are cast by the source for the target.

• All votes are, initially, considered equal.

• Over the course of executing the algorithm on a link graph, pages which receive more votes become more important.

• More important pages cast more important votes.

• The votes that a page can cast are a function of that page's importance, divided by the number of votes/links it casts.

That algorithm, of course, was named PageRank, and it changed the course of web search, providing ...

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