Google PageRank Primer - How is That PageRank Determined?
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PageRank is a form of a voting system. A link to a page is a vote for that page. Higher PageRank pages are viewed by Google as more important. Their votes are given more value by Google. In some cases, much more value.
Adding incoming links to your web pages will add PageRank. Every inbound link adds some PageRank, regardless of its own level. PageRank flows from one page to another, adding to its store of importance.
Not all incoming links provide the same inflow of value. It may take many PR2 incoming links to increase your targeted page to PR5. On the other hand, it might only take one PR6 page to give you the same result.
Google has published their original PR calculation formula:
While we are not certain if this formula is still the one used by Google, it is probably at least very similar.
In the formula, PR(A) is the PageRank of the page = (1- d) where d is a "damping factor considered to be about 0.85 + d(PR(t1) where t1 is the PR of the incoming link page + ...+ PR (tn) is the Page Rank of all of the linking pages. Each page is divided by C which is the number of outgoing links from each page.
Note that the PR flow from a page is divided equally between all of the links on the page. If a page has one outgoing link, that receiving page gets the entire flow. If there are ten links on the page, the PR flow is divided ten ways, lessened by the 0.85 damping factor.
Simply put the formula is this:
PR(A) = .15 + .85 * the PR share of every incoming link page.
Based on the formula, the more incoming links from higher PR pages the better. On the other hand, fewer outgoing links from the sending page, the better too. You get less PR if it’s divided among more outgoing links. Everyone gets their share of the PR pie. Your piece simply becomes smaller if there are more mouths to feed!
You can theoretically gain more total PR from a PR4 page, where you are the only recipient, than from a PR8 page divided 100 ways. The bottom line is to add, within the context of an overall linking program, as many incoming links as you can. They all add PR to varying degrees.
Google Optimization 
