PageRank is an algorithm which analyses the link structure of a hyperlinked set of webpages and then assigns a numerical weighting to each page as a way of measuring their relative importance within that set.  The PageRank of a particular webpage is an indication of its value within the Google index. The scale is a range from 0 to 10.

A page with high PageRank will have a direct positive effect on its position within search results, but it is not the only factor considered by the Google algorithm when ranking a page - it is only one of approximately 200 factors but it is, none the less, one of the most important.

This is how Google describes PageRank:

"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"."

As a website begins to increase in popularity via the quality and quantity of backlinks, the PageRank of a particular page also increases.  This increase in number however is based, we believe, on a logarithmic scale.  As an example if using a base 10 scale, to get a PageRank of 1 the page requires 10 backlinks pointing to it, then to gain another point on the scale, the page will require an extra 100 backlinks, to gain another point an extra 1000 backlinks will be required and so on.

Retaining all of that valuable PageRank can be tricky and a lot of it has to do with how the internal linking structure of your website is set up. It is very easy to leak ‘link juice’ from important pages into less important pages that are less likely to appear in search results.  There are methods available to search marketers that allow us to reduce this leakage commonly referred to as PageRank sculpting.

Another way of leaking ‘link juice’ is to have large amounts of external linking to other websites without getting a similar amount of links back into your website.  As part of our link building process we are constantly aware of checking the likely value of a link from a particular website before we leak any ‘link juice’ out to that site via a reciprocal link. The goal is to link with pages that have equal or higher PageRank to avoid the negative effect.

As you’ll understand by now, although PageRank is a critical component in Google’s ranking algorithm, due to it's logarithmic nature it is very difficult to obtain a high PageRank even after years of link building.  By it's very nature, the web changes and the page that a link comes from may not carry as much weight as it did when the link was first obtained, regardless of the PageRank measure for that page.  It could simply be that other pages that were ‘voting’ for that particular page may not carry as much weight as they once did which has a flow on effect to the value of the link going to your site.

Google advise that the PageRank assessment of a webpage is calculated on a regular basis but that the PageRank scale shown on the Google Toolbar is only updated 2-3 times per year so the very best option for all websites to gain rankings is to continue to build credible backlinks and not to be discouraged when the PageRank scale doesn’t slide upwards overnight.