Google's PageRank

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PageRank is Google's method of giving a numerical value to the number and quality of the links pointing to a web page. The PageRank of a page is based on a scale from 0 to 10, which is displayed on the Google Toolbar for Microsoft Internet Explorer and Firefox. It was the original foundation for Google's approach to ranking, as expressed in the doctoral thesis by the company's founders Larry Page and Sergey Brin when they attended Stanford. Their theory was that the quality of any webpage could, in part, be evaluated in terms of the number and quality of the links from other webpages that point to it. The theory equated each link as a "vote" for the page being linked to. But instead of a straight numerical count of these links, PageRank also considered the quality of each link.

The original PageRank formula: For any Page A which is linked to by pages T1 through Tn, the PageRank of Page A is determined by

PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))

where PR(A) is the PageRank of page "A", PR(Ti) is the PageRank of page "Ti", C(Ti) is the total number of links on page "Ti7", and d is a damping factor.

As you can see in the formula above, the quality of a link is based on the PageRank score of the page on which the link originates divided by the number of links on that same page. So, a link from a page with a modest PageRank score but which contains relatively few links can pass on more PageRank value than a link from a page with a relatively higher PageRank score that contains many links. Each web page's PageRank score is then the approximate sum of the PageRank value of the links pointing to it. The exact formula Google uses for determining PageRank has changed over the years, but the foundation seems to have remained essentially as I describe here. In addition, the scale on which PageRank operates is also subject to change, which is why you'll sometimes see the PageRank of your pages fall after an update even though the links pointing to that page appear to have remained unchanged.

The value of PageRank in affecting the ranking of a particular page is still quite high, but not as high as it used to be, which has caused many people to generalize that PageRank is irrelevant or worthless. Within the past couple of years, Google has been giving a great deal of value to the text used within the link's anchor text - that is, the text inserted between the and tags in the HTML code. But since you still need a link to get the value of the anchor text boost, which would also contribute to a page's PageRank score, the two are very connected in the ranking potential of every web page. In the end, while they are distinct ranking factors used by Google, you can't have one without the other.