Think of page rank as a cup that can be filled up. Once it’s filled, it can then fill up several other cups (or in this case, other websites). It does this through linking to other pages. Every link on a page can fill up another page a little bit, but no cup can be empty, so it retains a bit of its rank as well. The result is that the further a page is from an authoritative site in links, the less on that site’s value will trickle down to that page.
When page rank is passed, it is split evenly across all links, including no follow links— so, a page with 10 links, two of which are no follow links, would still pass only one tenth of its authority to each of the other eight pages. This means that adding no follow links to third-party sites does not help you retain page rank in your site. The only advantage a no follow may have is to deter spammers from posting many links in a public forum by limiting the no follows off-site.
To track page rank data, SEOmoz provides two tools: Domain mozRank and mozRank. The values these tools return are not numbers provided by any search engine; instead these are numbers that SEOmoz has developed that can be used to estimate the value of pages. This data is found in the Open Site Explorer report, which provides details about inbound links, number of linking root domains, and more.
This data becomes very useful when trying to understand how and why certain pages rank in the positions in which they do. These are yet more analytics you can put into a keyword research diagram to figure out exactly what your chances of a page ranking on a specific term are. Pagerank can be a very important factor, but domain rank is also important. For example, even if your site has the exact same content as Wikipedia, it is likely that Wikipedia will outrank you, simply because of the authority of its parent domain. To move past this top-ranking site, you would need to grow your page rank significantly beyond that of Wikipedia, to compensate for your less authoritative domain.
Tracking both you and your competitors’ page and domain ranks over time will also help you anticipate if a competitor may overtake you on specific key terms that are important to your overall search strategy. Anticipating issues before they arise will allow you to become proactive and ensure that you retain the ground you have gained, as well as understanding what ground you are close to gaining.
Source of Information : MASTERING SEARCH ANALYTICS MEASURING SEO, SEM AND SITE SEARCH