Here's a question we recently received: "Can you send us research or any articles you have regarding the theory behind inserting nofollows on links on a page, for example with the privacy policy or site map?"

The rel="nofollow" link attribute is designed to give webmaster granular control over the flow of link equity (PageRank).  Links that have the "nofollow" attribute end up passing zero equity, are not followed (crawled) and are dropped off the link graph of the major search engines.  Nofollow can be a great way to optimize your webpages to appear more relevant to the engines, and as a result, people.

If you think of each link on a web page as a valve that sends link equity down a pipe then you could shut off some of those valves and send additional equity down the pipes that matter the most.  Nofollowing is usually a good method of preventing link equity from propegating to pages that you don't want or need ranking in the serps.

How do I use it?

Simply place the rel="nofollow" attribute into the <a> tag

Without nofollow: <a href="http://www.example.com">my link here</a>

With nofollow: <a href="http://www.example.com" rel="nofollow">my link here</a>

When should I use Nofollow? Examples

• Unimportant pages - One idea would be to nofollow the privacy policy and terms & conditions pages on all pages of your site except the sitemap, then nofollow the sitemap on all pages except the home page - so you'd be saving equity all over the site but still letting the crawlers find your sitemap, from the home page, and all pages it links to.
   
• Duplicate links - You could make sure you don't pass link equity to the same page twice by placing using nofollow on duplicate links.  Note you may not want to always follow this rule if the cost of implementing nofollow attributes on duplicate links is higher than the perceived benefit (for example, I would probably avoid trying to write rules in your CMS that place a nofollow attribute on a link pointing to page "A" within your navigation if a contextual link on that same page is linking to page "A" as well... just too messy) - and be careful because it's easy to shoot yourself in the footer since feedback on nofollow difficult to measure other than ranking changes. 

  A really conservative and safe approach would be to nofollow duplicated links within the copy of your page, so if you sold legal related services and you linked to your services page more than once within your page copy you might try nofollowing the link that says "services" and keep the "San Diego Family Lawyer" link followed.
   

• Paid links and ads - Nofollowing paid advertisements is probably a good idea, however I'm sure many will argue against it.  Search engines want you to nofollow paid links because they argue paid links unfairly re-route equity and do not represent natural linking, which reduces an engine's ability to serve up relevant organic results. Google for example even have a way for you to report paid links.
   
• Comment links - it's no secret that nofollow is used to reduce the effect of link equity, PageRank etc. from leaking away to sites that you don't necessarily want your page recommending, especially if you let people post comments freely without moderation.
   
• When you complain - that's right, when you complain about someone or something and you feel you need to write about it, why not link to your example but not give out link equity at the sime time?  Sounds good because they didn't deserve it anyways right? ;)

Tools for working with Nofollow tags

• NoDoFollow FireFox pluggin - A simple pluggin for FireFox that, when enabled, highlights all links on a page in blue and all nofollowed links in pink making it easy to eye ball a page and see what it's doing.  Try view this blog page for nofollow links, we've done a few minor tweaks here and there.
•  Performing an anchor text analysis on our tool, built on top of backlinkwatch.com, will show nofollowed links (just look in the "Flag" column)

Alternatives to using Nofollow

• A Meta tag nofollow can be used to prevent all links on a given page from sending any equity away from your page
• And you could use a few Robots exclusion rules to prevent entire sections of your site from sending out link "juice".

Be careful with the above two because you can nofollow or exclude an entire page and still have other, indexed pages send equity down into it, wasting your resources - and the nofollow tag is great because you get granular control over the flow of PageRank and don't necessarily need to worry about other pages waisting equity on your page in question.

What about opinions against the use of NoFollow?

Apparently the smaller the player, the riskier your online business model or the more negatively charged reputation you have with a search engine, the benefits of using nofllow seem to grow smaller and smaller, especially with respect to Google (because their quality raters have the power to filter/ban your site):

"If you are a corporation you are OK to spam, but Google engineers get their kicks by crushing small webmasters. They keep the web safe and healthy by policing smaller players, hoping that this consolidates traffic against larger corporations which will grow dependant on Google traffic, and will end up easily swayed by Google’s business interests. Short term the corporations are not well policed so that they can become addicted to free traffic, but after many of the smaller players have been driven off the web (largely in the next 5 years) Google will start policing the profits out of corporations too."

Where can I learn more about nofollow?

Google has a nofollow help page here:
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=96569

Matt Cutts is Google's head of web spam and has a few thoughts on the matter.