Saturday, October 20, 2007

PageRank

From Wikipedia, the free encyclopedia

PageRank is a link analysis algorithm that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set. The algorithm may be applied to any collection of entities with reciprocal quotations and references. The numerical weight that it assigns to any given element E is also called the PageRank of E and denoted by PR(E).

The name PageRank is a trademark of Google. The PageRank process has been patented (U.S. Patent 6,285,999 ). The patent is not assigned to Google but to Stanford University

General description
Google describes PageRank:[1]

“ 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". ”

In other words, a PageRank results from a "ballot" among all the other pages on the World Wide Web about how important a page is. A hyperlink to a page counts as a vote of support. The PageRank of a page is defined recursively and depends on the number and PageRank metric of all pages that link to it ("incoming links"). A page that is linked to by many pages with high PageRank receives a high rank itself. If there are no links to a web page there is no support for that page.

Google assigns a numeric weighting from 0-10 for each webpage on the Internet; this PageRank denotes a site’s importance in the eyes of Google. The scale for PageRank is logarithmic like the Richter Scale and roughly based upon quantity of inbound links as well as importance of the page providing the link.

Numerous academic papers concerning PageRank have been published since Page and Brin's original paper.[2] In practice, the PageRank concept has proven to be vulnerable to manipulation, and extensive research has been devoted to identifying falsely inflated PageRank and ways to ignore links from documents with falsely inflated PageRank.

Alternatives to the PageRank algorithm include the HITS algorithm proposed by Jon Kleinberg, the IBM CLEVER project and the TrustRank algorithm.


PageRank variations

[edit] Google Toolbar

An example of the PageRank indicator as found on the Google toolbar.The Google Toolbar's PageRank feature displays a visited page's PageRank as a whole number between 0 and 10. The most popular websites have a PageRank of 10. The least have a PageRank of 0. Google has not disclosed the precise method for determining a Toolbar PageRank value. Google representative Matt Cutts has publicly indicated that the Toolbar PageRank values are republished about once every three months, indicating that the Toolbar PageRank values are historical rather than real-time values.[6]


[edit] Google directory PageRank
The Google Directory PageRank is an 8-unit measurement. These values can be viewed in the Google Directory. Unlike the Google Toolbar which shows the PageRank value by a mouseover of the greenbar, the Google Directory does not show the PageRank as a numeric value but only as a green bar.


[edit] False or spoofed PageRank
While the PR shown in the Toolbar is considered to be derived from an accurate PageRank value (at some time prior to the time of publication by Google) for most sites, it must be noted that this value is also easily manipulated. A current flaw is that any low PageRank page that is redirected, via a 302 server header or a "Refresh" meta tag, to a high PR page causes the lower PR page to acquire the PR of the destination page. In theory a new, PR0 page with no incoming links can be redirected to the Google home page - which is a PR 10 - and by the next PageRank update the PR of the new page will be upgraded to a PR10. This spoofing technique, also known as 302 Google Jacking, is a known failing or bug in the system. Any page's PR can be spoofed to a higher or lower number of the webmaster's choice and only Google has access to the real PR of the page. Spoofing is generally detected by running a Google search for a URL with questionable PR, as the results will display the URL of an entirely different site (the one redirected to) in its results.


[edit] Manipulating PageRank
For search-engine optimization purposes, some companies offer to sell high PageRank links to webmasters.[7] As links from higher-PR pages are believed to be more valuable, they tend to be more expensive. It can be an effective and viable marketing strategy to buy link advertisements on content pages of quality and relevant sites to drive traffic and increase a webmaster's link popularity. However, Google has publicly warned webmasters that if they are or were discovered to be selling links for the purpose of conferring PageRank and reputation, their links will be devalued (ignored in the calculation of other pages' PageRanks). The practice of buying and selling links is intensely debated across the Webmastering community. Google advises webmasters to use the nofollow HTML attribute value on sponsored links. According to Matt Cutts, Google is concerned about webmasters who try to game the system, and thereby reduce the quality of Google search results.[7]


[edit] Other uses of PageRank
A version of PageRank has recently been proposed as a replacement for the traditional ISI impact factor,[8] and implemented at eigenfactor.org. Instead of merely counting total citation to a journal, the "importance" of each citation is determined in a PageRank fashion.

A similar new use of PageRank is to rank academic doctoral programs based on their records of placing their graduates in faculty positions. In PageRank terms, academic departments link to each other by hiring their faculty from each other (and from themselves).[9]

PageRank has also been used to automatically rank WordNet synsets according to how strongly they possess a given semantic property, such as positivity or negativity.[10]

A dynamic weighting method similar to PageRank has been used to generate customized reading lists based on the link structure of Wikipedia.[11]

A Web crawler may use PageRank as one of a number of importance metrics it uses to determine which URL to visit next during a crawl of the web. One of the early working papers[12] which were used in the creation of Google is Efficient crawling through URL ordering,[13] which discusses the use of a number of different importance metrics to determine how deeply, and how much of a site Google will crawl. PageRank is presented as one of a number of these importance metrics, though there are others listed such as the number of inbound and outbound links for a URL, and the distance from the root directory on a site to the URL.


[edit] Google's "rel='nofollow'" proposal
In early 2005, Google implemented a new value, "nofollow", for the rel attribute of HTML link and anchor elements, so that website builders and bloggers can make links that Google will not consider for the purposes of PageRank — they are links that no longer constitute a "vote" in the PageRank system. The nofollow relationship was added in an attempt to help combat spamdexing.

As an example, people could create many message-board posts with links to their website to artificially inflate their PageRank. Now, however, the message-board administrator can modify the code to automatically insert "rel='nofollow'" to all hyperlinks in posts, thus preventing PageRank from being affected by those particular posts.

This method of avoidance, however, also has various drawbacks, such as reducing the link value of actual comments. (See: Spam in blogs#rel="nofollow")

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