What Is Google Page Rank?
Google Page Rank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote for the other page.
The more votes that are cast for a page, the more important the page must be. Also, the importance of the page that is casting the vote determines how important the vote itself is. Google calculates a page's importance from the votes cast for it. How important each vote is is taken into account when a page's Google Page Rank is calculated.
Google Page Rank is Google's way of deciding a page's importance. It matters because it is one of the factors that determines a page's ranking in the search results. It isn't the only factor that Google uses to rank pages, but it is an important one.
Google Page Rank Tips Domain names and Filenames
To a spider,
www.domain.com/, domain.com/,
www.domain.com/index.html and domain.com/index.html are different urls and, therefore, different pages. Surfers arrive at the site's home page whichever of the urls are used, but spiders see them as individual urls, and it makes a difference when working out the Google Page Rank. It is better to standardize the url you use for the site's home page. Otherwise each url can end up with a different Google Page Rank, whereas all of it should have gone to just one url.
If you think about it, how can a spider know the filename of the page that it gets back when requesting
www.domain.com/ ? It can't. The filename could be index.html, index.htm, index.php, default.html, etc. The spider doesn't know. If you link to index.html within the site, the spider could compare the 2 pages but that seems unlikely. So they are 2 urls and each receives Google Page Rank from inbound links. Standardizing the home page's url ensures that the Google Page Rank it is due isn't shared with ghost urls.
See the article on
Canonical URLs in our knowledgebase.
Imagine the page,
www.domain.com/index.html. The index page contains links to several relative urls; e.g. products.html and details.html. The spider sees those urls as
www.domain.com/products.html and
www.domain.com/details.html. Now let's add an absolute url for another page, only this time we'll leave out the "www." part - domain.com/anotherpage.html. This page links back to the index.html page, so the spider sees the index pages as domain.com/index.html. Although it's the same index page as the first one, to a spider, it is a different page because it's on a different domain.
Now look what happens. Each of the relative urls on the index page is also different because it belongs to the domain.com/ domain. Consequently, the link stucture is wasting a site's potential Google Page Rank by spreading it between ghost pages.
How is Google Page Rank calculated?
To calculate the Google Page Rank for a page, all of its inbound links are taken into account. These are links from within the site and links from outside the site.
PR(A) = (1-d) + d(PR(t1)/C(t1) + ... + PR(tn)/C(tn))
That's the equation that calculates a page's Google Page Rank. It's the original one that was published when Google Page Rank was being developed, and it is probable that Google uses a variation of it but they aren't telling us what it is. It doesn't matter though, as this equation is good enough.
In the equation 't1 - tn' are pages linking to page A, 'C' is the number of outbound links that a page has and 'd' is a damping factor, usually set to 0.85.
We can think of it in a simpler way:-
A page's Google Page Rank = 0.15 + 0.85 * (a "share" of the Google Page Rank of every page that links to it)
"share" = the linking page's Google Page Rank divided by the number of outbound links on the page.
A page "votes" an amount of Google Page Rank onto each page that it links to. The amount of Google Page Rank that it has to vote with is a little less than its own Google Page Rank value (its own value * 0.85). This value is shared equally between all the pages that it links to.
From this, we could conclude that a link from a page with PR4 and 5 outbound links is worth more than a link from a page with PR8 and 100 outbound links. The Google Page Rank of a page that links to yours is important but the number of links on that page is also important. The more links there are on a page, the less Google Page Rank value your page will receive from it.
If the Google Page Rank value differences between PR1, PR2,.....PR10 were equal then that conclusion would hold up, but many people believe that the values between PR1 and PR10 (the maximum) are set on a logarithmic scale, and there is very good reason for believing it. Nobody outside Google knows for sure one way or the other, but the chances are high that the scale is logarithmic, or similar.
If so, it means that it takes a lot more additional Google Page Rank for a page to move up to the next Google Page Rank level that it did to move up from the previous Google Page Rank level. The result is that it reverses the previous conclusion, so that a link from a PR8 page that has lots of outbound links is worth more than a link from a PR4 page that has only a few outbound links.
Whichever scale Google uses, we can be sure of one thing. A link from another site increases our site's Google Page Rank. Just remember to avoid links from link farms.
Note that when a page votes its Google Page Rank value to other pages, its own Google Page Rank is not reduced by the value that it is voting. The page doing the voting doesn't give away its Google Page Rank and end up with nothing. It isn't a transfer of Google Page Rank. It is simply a vote according to the page's Google Page Rank value. It's like a shareholders meeting where each shareholder votes according to the number of shares held, but the shares themselves aren't given away.
Even so, pages do lose some Google Page Rank indirectly, as we'll see later.
Ok so far? Good. Now we'll look at how the calculations are actually done.
For a page's calculation, its existing Google Page Rank (if it has any) is abandoned completely and a fresh calculation is done where the page relies solely on the Google Page Rank "voted" for it by its current inbound links, which may have changed since the last time the page's Google Page Rank was calculated.
The equation shows clearly how a page's Google Page Rank is arrived at. But what isn't immediately obvious is that it can't work if the calculation is done just once. Suppose we have 2 pages, A and B, which link to each other, and neither have any other links of any kind. This is what happens:-
Step 1: Calculate page A's Google Page Rank from the value of its inbound links
Page A now has a new Google Page Rank value. The calculation used the value of the inbound link from page B. But page B has an inbound link (from page A) and its new Google Page Rank value hasn't been worked out yet, so page A's new Google Page Rank value is based on inaccurate data and can't be accurate.
Step 2: Calculate page B's Google Page Rank from the value of its inbound links
Page B now has a new Google Page Rank value, but it can't be accurate because the calculation used the new Google Page Rank value of the inbound link from page A, which is inaccurate.
It's a Catch 22 situation. We can't work out A's Google Page Rank until we know B's Google Page Rank, and we can't work out B's Google Page Rank until we know A's Google Page Rank.
Now that both pages have newly calculated Google Page Rank values, can't we just run the calculations again to arrive at accurate values? No. We can run the calculations again using the new values and the results will be more accurate, but we will always be using inaccurate values for the calculations, so the results will always be inaccurate.
The problem is overcome by repeating the calculations many times. Each time produces slightly more accurate values. In fact, total accuracy can never be achieved because the calculations are always based on inaccurate values. 40 to 50 iterations are sufficient to reach a point where any further iterations wouldn't produce enough of a change to the values to matter. This is precisiely what Google does at each update, and it's the reason why the updates take so long.
One thing to bear in mind is that the results we get from the calculations are proportions. The figures must then be set against a scale (known only to Google) to arrive at each page's actual Google Page Rank. Even so, we can use the calculations to channel the Google Page Rank within a site around its pages so that certain pages receive a higher proportion of it than others.
The Google toolbar
If you have the Google toolbar installed in your browser, you will be used to seeing each page's Google Page Rank as you browse the web. But all isn't always as it seems. Many pages that Google displays the Google Page Rank for haven't been indexed in Google and certainly don't have any Google Page Rank in their own right. What is happening is that one or more pages on the site have been indexed and a Google Page Rank has been calculated. The Google Page Rank figure for the site's pages that haven't been indexed is allocated on the fly - just for your toolbar. The Google Page Rank itself doesn't exist.
It's important to know this so that you can avoid exchanging links with pages that really don't have any Google Page Rank of their own. Before making exchanges, search for the page on Google to make sure that it is indexed.
Sub-directories
Some people believe that Google drops a page's Google Page Rank by a value of 1 for each sub-directory level below the root directory. E.g. if the value of pages in the root directory is generally around 4, then pages in the next directory level down will be generally around 3, and so on down the levels. Other people (including me) don't accept that at all. Either way, because some spiders tend to avoid deep sub-directories, it is generally considered to be beneficial to keep directory structures shallow (directories one or two levels below the root).
ODP and Yahoo!
It used to be thought that Google gave a Google Page Rank boost to sites that are listed in the Yahoo! and ODP (a.k.a. DMOZ) directories, but these days general opinion is that they don't. There is certainly a Google Page Rank gain for sites that are listed in those directories, but the reason for it is now thought to be this:-
Google spiders the directories just like any other site and their pages have decent Google Page Rank and so they are good inbound links to have. In the case of the ODP, Google's directory is a copy of the ODP directory. Each time that sites are added and dropped from the ODP, they are added and dropped from Google's directory when they next update it. The entry in Google's directory is yet another good, Google Page Rank boosting, inbound link. Also, the ODP data is used for searches on a myriad of websites - more inbound links!
What is Pagerank? 
Linear Mode
