The Page Rank Algorithm
Google Page Rank is a system used by Google for measuring the number and quality of links onto a web site. Originally defined in a thesis by the Google founders, Sergey Brin and Larry Page, Page Rank has become the source of much confusion and myth. Many webmasters and SEOs are obsessed with Page Rank and view it as a key measure of success. This has led to a lot of wasted time and activity. This article will discuss what Page Rank is, how it works, and how it matters.
The original Page-Brin thesis defined Page Rank in a precise mathematical manner. In basic terms, every single page that links to another page (we will call it “Page A”) counts as a vote for Page A. If the linking page has a Page Rank of 1, and links to 5 total pages, each of these pages, including Page A, is getting a vote of 0.2. If the linking page has a Page Rank of 2, and links to 5 total page, each of these pages, including Page A, is getting a vote of 0.4.
So now let’s complicate things a bit. The Page Rank you see in the Google toolbar is not the form of “Raw Page Rank” used in these calculations. Raw Page Rank as calculated in the prior paragraph grows to be a very large number, certainly much larger than the number 10 that is the largest number that you might see in the Google toolbar. The Page Rank that people talk about for sites is a logarithmic version of the Raw Page Rank. So if the Raw Page Rank is equal to 10 to the 7th power (10,000,000), this is roughly equivalent to a PR7 site.
Speaking a bit more precisely, the exact algorithm is not known, but we do know that Google takes the known universe of sites, and then allocates their Raw Page Rank into 10 buckets, ranging from Page Rank 1 (“PR1”) to PR10. As stated above, we do know that the original algorithm was logarithmic, so we can estimate that a PR6 site has 10 times as much Raw Page Rank as a PR5 site.
So now let’s complicate things a bit further. Let’s say that we are about to recalculate the Raw Page Rank of the web. Before doing the recalc, Page B has a Raw Page Rank of 20, and it links to 10 sites, including Page A. So in calculating the value to Page A during this recalc, we determine that Page B is passing on 2 Raw Page Rank points to Page A.
However, after finishing the recalc of the Raw Page Rank value for all the sites on the web, we find out that the Raw Page Rank of B has changed – now it has a raw Page Rank of 30. So the value of the link to Page A is now 3 points. This leads us to recalculate again, but after this calculation, Page B’s page rank has moved up to 35 … How does Google handle this? Google does the calculations over and over (“recursively”) until the level at which the Raw Page Rank values are changing is acceptably small.
How is Page Rank Used?
In the early days of Google, Page Rank was a large factor in determining search engine placement. Pages with a high Page Rank were likely to place highly for the search terms relevant to the page. This led to a whole host of techniques used by Spammers to get their sites to rank highly in the search results. People purchased links from other sites, and they swapped links in high volume. Some spammers even came up with tricks using 302 redirects to hijack traffic from other sites by capitalizing on their Page Rank.
Slowly, but steadily, Google adapted. They figured out how to recognize swapped (“reciprocal”) links and to discount them. They have steadily gotten better at recognizing purchased links and have figured out how to discount them. Note that they do this because swapped links and purchased links do not represent true endorsements of the pages that they link to, causing the algorithm to fail.
In addition, Google, and all the search engines, began to figure out that links from highly relevant sites were a better indicator of the sites that would provide the best search results for a particular search query. Their algorithms have shifted dramatically in the direction of weighting links based on relevance. In very crude terms, you can think of a major used car site having a link value of “PR7” to another used car site and a link value of a “PR1” to a site about balloons.
In conclusion, the Page Rank value that you can see in a Google toolbar has little to do with ranking your site today. There are reportedly more than 100 factors used by Google in determining how a site ranks for a particular search term. Traditional Page Rank is but one of these, and certainly not one of the biggest. The biggest factors relate to how your site design makes it easy for Google to determine what your site is about, and the number and quality of relevant sites that link to you (in particular, when compared to your competitors).
Page Rank does influence your success in three ways that remain significant:
- Google crawls the web in Page Rank order, starting with PR10 sites, and working their way down.
- Google crawls higher PR sites more often, and more deeply.
- Google will index more pages from a high PR site (this is very important if you have a site with a large number of pages).
These are the three most important things you gain by having high Page Rank. Admittedly, these are pretty important. But, the best way to build up your overall Page Rank is by getting links from more and more relevant sites. That way you win the rankings game at the same time. Read these articles for more information on strategies for getting people to link to your site: