Pagerank is often confused with link popularity but although related they are far from the same. Let's start with PageRank. The origins of this metric are by no means rooted in the Internet. Instead, similar ranking systems have been in long time use in academic and scholarly environments in the form of a citation index. In a citation index, the importance of an academic (scientific) paper or article is determined by the number of other pages or articles that refer to it. Papers that are often cited or quoted are therefore deemed more important and relevant to their subject than those that get less frequently quoted.

It's no coincidence that Larry Page and Sergey Brin, the founding fathers of Google, first cobbled together an embryonic version of the famous Search Engine, during their time at Stanford University. The precursor of Google which they designed was intended for indexing academic documents of Stanford's intranet. They later formalised the algorithm of the citation index they used for determining the index, by patenting the algorithm as PageRank.

At the heart of this technology lies an algorithm that may seem awe inspiring to the mathematically uninitiated but which in essence is very easy to understand when broken down in bite size chunks. The algorithm strives to determine the importance of an individual Web page by calculating a single number based on the number and the quality of the pages that link to it. How important is this metric in respect of your page ranking? The clue is in the name: very important indeed, more about that later. Webmasters often refer to pages that link to their own as inbound links. How to obtain inbound links and how to distribute them over the pages of your Website is the subject or our "linking Strategies" page. But for now, let's try and understand what PageRank is all about.

A little basic maths will have to be deployed here. Bear with us: this little journey will definitely be worth the time spent on it and help you get PageRank for your pages. Understanding PageRank better is the only possible basis for choosing an effective linking policy and improving your pages' Search Engine rankings.

When a page (say A) links to another page (say B), the latter page has been given a "vote" by the former. The effect of this vote is that B will have received an amount of PageRank which here we'll call PA to B. The amount PA to B, according to Google's algorithm, depends on two factors:

1. The actual PageRank the page A currently possesses, call it PA. The higher the value of PA, the more page A will contribute to the PageRank of Page B. In plain English: the vote received by page B will count for more if it comes from a page that itself has high importance.
2. The number of votes cast by page A: a higher number of votes means each vote confers less PageRank. In plain English, the higher the number of outbound links page A has the less it will contribute to its partners PageRank. We'll call the number of outbound links of page A, m.

According to the algorithm the contribution of page A to page B's PageRank P_{A to B} is:

P_{A to B} = P_A / m

P_{i to B} = P_i / m_i

For reasons that are definitely outside the scope of this page, the Google algorithm doesn't just summate all these contributions but introduces an additional element known as the damping factor d, which is believed to be set to about 0.85. Using the above and the damping factor the total PR value of a page is then calculated as:

P = (1-d) + d.(P₁ / m₁) + ... + d.(P_i / m_i) + ... + d.(P_n / m_n) for i from 1 to n

or: P = (1-d) + d.SUM(P_i / m_i) for i=1 to i=n

Before we go any further, it's important to note that the number thus obtained is not what is displayed in the Google's toolbar (if you haven't already installed the Google toolbar, go to the Google toolbar page to download and install it: it's free and really useful). The value that is displayed in the Google toolbar and which is commonly referred to as PR appears to be on a logarithmic scale. Below you'll find the real TR values, according to toolbar PR (assuming log base 10)

Others believe the toolbar value to be logarithmic with a log base of 6 to 7.

A few important observations can already be made:

1. Just about everybody refers to the "PR of a Website" but Websites have no PR: only individual pages have PR. Throughout the pages of any Website, the PR will vary strongly from page to page. This distribution will depend on how you organise your site internally and what linking strategy you deploy. The right internal structure and linking policy can ensure that you get PageRank that isn't just concentrated on your home page. Concentrating all PR on the home page is only useful if the home page is also the main landing page.
2. Not all links are born equal: up to relatively recently Webmasters used to measure their inbound links by means of link popularity, a simple count of the number of pages linking to your own page. The amount a particular link will contribute to your page's PR depends strongly on the page that cast the vote and will vary from "quite a lot" to "almost nothing". Link popularity is a useful metric but it doesn't quite tell the whole story. You can download a free link popularity checker from the toolbox. To get Pagerank to your pages, you'll need inbound links from pages that have some PR themselves and don't distribute it to hundreds of link partners.