I found my PR became "NA" (N/A). What is NA?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Charles Lee: Hello Friends! Need some assistance here.
  
  ok, I have not really been constantly checking the Google PR for my blog, but I would do that once in a while
  
  It started with a PR3, about a few months into blogging. A year later, it went down to PR2, which I thought may be fine too.
  
  However, just now, about 2 years later into blogging, I found that the PR became "NA" (N/A)
  
  ok, I've learnt that if i need to find info or facts on the internet, I usually try to check it from two different sources. I used two PR checker that I found from googling it, and both gives me the same "NA" result
  
  Is my blog in trouble?
  
  (blog is: SingaporeanLifeStyle dot com)
  
  I have not been constantly updating the blog, but I thought it would drop to 1/10 or maybe even a "0", but what's N/A ?
  
  Thanks!
• 
  Luke Ciciliano: Thanks for sharing +Charles Lee . It's important to stress that page rank is only one thing Google looks at and updating your blog regularly is just as, if not more, important. Keep in mind that page rank is the result of other relevant sites providing you with quality links and the score is influenced by quite a few things (for example - if the score of the sites linking to you drop then your score drops as a result). You may want to make sure your updating your blog regularly as a starting point (just a suggestion). ;
• 
  Ranu Jain: Yes, Pagerank is the Google’s way of measuring how popular a website is; based on linkage data. Higher quality backlinks and the amount of them translate into a high Pagerank. In case you want to delve deeper into mathematical understanding (not needed really), here is the formula:
  
  PR (A) = (1-d) + d (......) to PR (A) = PR (A) + (1-d) + d (......) ;
   ;  ;  ;  ;  ;  ;
  It means
  
  A page's PageRank = 0.15 + 0.85 * (a "share" of the PageRank of every page that links to it)
  
  Study your blog's link profile and build some quality and relevant links. ;
  
  Well, another point to remember that there has been no PR update since last six months. ;
  
  In fact, webmasters are questioning whether PR is still important or no more an important factor. ;
• 
  Guillaume Boucher Bélanger: Here is something you should read : http://searchenginewatch.com/article/2302895/Matt-Cutts-on-SEO-PageRank-Spam-the-Future-of-Google-Search-at-Pubcon-Las-Vegas