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PageRank

PageRank is the concept that enabled Google to leap-frog over other search engines during the late 1990s in terms of relevant search engine results pages and speed of search. PageRank holds US patent number 6,285,999 granted on the 4th September 2001 to Larry Page, one of Google's founders. Page's idea is based on established practice for scientific papers where the importance of a paper is based on the number of citations made to it by other papers. The Google algorithm interprets a link from one page to another as a vote. The Google algorithm and the configuration of web pages means that page rank can be fed back so that pages that have more votes are then deemed to be more important and the votes they cast (outbound-links) are subsequently given more weight.

Many web designers' efforts to optimize their sites for search engines, if they make any efforts at all, stop at acquiring inbound-links and exchanging links with other websites in the hope that this will boost their site's PageRank. This focus on PageRank is probably due in part to the Google toolbar that can be installed on Microsoft's Internet Explorer web browser. This shows a PageRank value for the current site on a scale of 0 to 10.

Getting a good site ranking is not quite as simple as getting a good score on the Google toolbar and decisions about outbound-links and site structure can have significant effects on PageRank.

Fortunately because of the patent the theory behind PageRank is known. The formula is relatively simple:

Page Rank Formula

Where

∂       damping factor, given as 0.85
n       total number of inbound links
PRⁿ    page rank of inbound page n
Cⁿ      number of outbound links from page n

So the PageRank (PR) for any page is equal to some constant value (0.15) plus the sum of all the page ranks from inbound links divided by the number of links (C) on each corresponding page multiplied by the damping factor (0.85).

Don't worry if that is too hard to visualize, we will look at some real examples below. One more thing before we do. As pages can, and usually do, feedback to other pages by way of hyperlinks the calculation has to be made iteratively. Every time Google visits a page it recalculates the PageRank based on the weights of the inbound links. Assuming no other changes it takes around 40 iterations for the figure to converge to a stable value although Google itself uses linear algebra to reduce the number of calculations. The damping factor (∂) is set at 0.85 for optimum performance.

PageRank

See Also