Google PageRank Primer - What is the Difference in PageRank Importance?
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The PageRank number shown on the Google Toolbar, is thought by many people, to be merely an estimate of a page’s true PageRank. While there is no real proof, one way or the other about the accuracy of the PageRank as presented, we can only assume is a good estimate. Only Google knows the actual PageRank, and they aren’t talking.
PageRank is not a series of equal steps; it is logarithmic in its calculation. In the same way that the earthquake Richter scale is exponential in calculation, so too is the mathematics behind Google PageRank. It takes one step to from a PR0 to a PR1. It takes a few more steps to PR2 and PR3. Now, it takes even more steps to PR4, many more steps again to PR5, and so on. Each level is progressively harder to reach.
Let’s consider an example.
For illustration purposes, think of a PR5 page. It is indicated by the PageRank meter being half green and half white. What does that mean?
Google PageRank is not a series of stepped numbers, as presented on the Toolbar. It is more of a continuum where our PR5 example may be barely over the boundary from being a PR4, or almost but not quite, a PR6 page. There is a lot of territory in between.
For our example page to reach a PR6, it will take many more incoming links, and perhaps higher PR pages as well, to reach that new higher level. It will be much more difficult to increase from a PR5 to a PR6, than the move was from PR4 to PR5.
Google Optimization 
