The PR of your site is updated about once of month when Google updates their database of web pages.
For example if you have a backward link from a site that has a PR of 5, some of that PR be will transfered from their website to your website page they linked to. These links are called backlinks.Sites with high PageRank will get crawled by the search engine bots more often, and the crawls will be deeper.
While Google takes into consideration the PageRank of your site, high PR doesn't always equate to a higher Search Engine Results Position (SERP). You can have high PR and have a low SERP on a certian keyword phrase, while some other site can have low PR but have a high SERP on the same keyword phrase.
In your pursue for a high PR keep in mind that numerous relevant keyword anchor texted backlinks will get you a higher SERP than just raw high PR and that with numerous relevant keyword anchor texted backlinks will come PR.
You aim shouldn't be just for high PR, but for proper backlinks that will bring with them a higher PR.
Page Rank Algorithm:
The following will give you an idea of how many inbound links and what PR those inbound links need to be, for you to obtain the desired PageRank for your pages. The following pagerank algorithm equation is the relationship between two pages A and B, with the below example page A receiving an inbound link from page B.
PR-A = (0.15) + (0.85 x PR-B / TOL-B)
PR-A is the PageRank of Page A, PR-B is the PageRank of Page B, and TOL-B is the Total Outbound Links of Page B.
For example if you get a inbound link from a page that has a PR of 6 and on that page there is a total of 6 outbound links (counting the one to your page), a PR of 1 will be added to your page: 0.15 + (0.85 x 6/6) = 1. If that one inbound link adds a PR of 1 to your page, and you wanted a PR of 5 it would take 5 similar inbound links.
By similar I mean pages that have a PR/TOL ratio of 1. That could be a PR of 5 with 5 outbound links, or a PR of 7 with 7 out bound links and so on. A PR/TOL ratio of 1 will add to your page a PR of 1. Using the PR/TOL ratio simplifies the RageRank algorithm equation and makes it easier for you to evaulate what PR a inbound link will add to your page.
When you are looking for inbound links just divide the PR of the page by its outbound links and this will give you a general idea of the PR a inbound link from that page would give you.
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