The most well-known system for ranking sites based on link data is the simplistic formula developed by Google’s founders - PageRank. PageRank, which relies on a mathematical formula (based around finding a given document in a random pattern of clicking on links), is described by Google in their technology section:
PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page’s value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves “important” weigh more heavily and help to make other pages “important.”
PageRank is, in essence, a rough system for estimating the value of a given link based on the links that point to the host page. Since PageRank’s inception in the late ’90s, more subtle and sophisticated link analysis systems have taken the place of PageRank. Thus, in the modern era of SEO, the PageRank measurement in Google’s toolbar, directory, or through sites that query the service is of limited value. Pages with PR8 can be found ranked 20-30 positions below pages with a PR3 or PR4. In addition, the toolbar numbers are updated only every 3-6 months by Google, making the values even less useful. Rather than focusing on PageRank, it’s important to think holistically about a link’s worth.
Here’s a small list of the most important factors search engines look at when attempting to value a link:
* The Anchor Text of Link - Anchor text describes the visible characters and words that hyperlink to another document or location on the web.
* Global Popularity of the Site - More popular sites, as denoted by the number and power of the links pointing to them, provide more powerful links. .
* Popularity of Site in Relevant Communities - In the example above, the weight or power of a site’s vote is based on its raw popularity across the web.
* Text Directly Surrounding the Link - Search engines have been noted to weight the text directly surrounding a link with greater important and relevant than the other text on the page. Thus, a link from inside an on-topic paragraph may carry greater weight than a link in the sidebar or footer.
* Subject Matter of the Linking Page - The topical relationship between the subject of a given page and the sites/pages linked to on it may also factor into the value a search engine assigns to that link. Thus, it will be more valuable to have links from pages that are related to the site/page’s subject matter than those that have little to do with the topic.
Link metrics are in place so that search engines can find information to trust. In the academic world, greater citation meant greater importance, but in a commercial environment, manipulation and conflicting interests interfere with the purity of citation-based measurements. Thus, on the modern WWW, the source, style, and context of those citations is vital to ensuring high quality results.
“Nofollow” is a tag designed specifically for search engines. When ascribed to a link in the rel attribute, it tells the engine’s ranking system that the link should not be considered an editorially approved “vote” for the linked-to page. Currently, 3 major search engines (Yahoo!, MSN, & Google) all support “nofollow”. AskJeeves, due to its unique ranking system, does not support nofollow, and ignores its presence in link code.