The Nofollow link element was originally designed as a tool to help webmasters deal with article and comment spam.
It was originally intended for application on links within blog and article comments.
These comments typically include a link to the commenter’s Web site of choice.
To combat manipulation, Google introduced the Nofollow attribute as a way for site managers to indicate a link to an untrusted site.
This original use for Nofollow still applies.
Another use of the Nofollow attribute is where paid links are concerned.
Google states:
Buying or selling links that pass PageRank is in violation of Google’s webmaster guidelines and can negatively impact a site’s ranking in search results.
Using the Nofollow attribute:
Adding a rel=”nofollow” attribute to the <a> tag will turn a link into a Nofollow link
PageRank is Google’s name for the value which passes from one page to another through a link.
In the following examples points are imaginary as Google’s algorithm is unknown.
They are just to give you an idea of the principles involved in PageRank distribution.
pagerank distribution
In the diagram on the right you will see that the PageRank is distributed equally through the links on the page.
If we add the Nofollow attribute to one of the links on the page it changes the equation.
PRsculpting
Previously, including Nofollow on a link would eliminate PageRank passing to that page and redistribute the flow of PageRank to the remaining links.
Previously, with Nofollow you could Prsculpt where you wanted the PR to go in your site
Today the Nofollow element behaves differently.
pagerank evaporation
A Nofollow element on a link evaporates the PageRank value, reducing the amount of PageRank that page can transfer through links.
Here we are assuming total evaporation.
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I wonder if it disappears completely or partially. To me, partial would seem to be more logical. Otherwise a page with many nofollow links (let’s say a popular blog post) would end up virtually losing the ability to pass on any pagerank at all.
This would go against quality theory as actual links in the content of a popular blog post would be left with little authority to pass on while the links in the content of an unpopular blog post could pass on more authority.
My guess is that the use of ‘evaporate’ as opposed to ‘remove’ or some other strong term suggests that it’s probably takes into account various factors such as the ratio of nofollow to follow links and probably the location on the page (eg. comments are typically at the bottom of a page so nofollow’s there would be treated differently than nofollows higher up in the content).
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