Hi Mike, OSE is a great tool, but believing it blindly is not always an good idea. An instance of error found in OSE PA. see below:
I serached a site PA through opensiteexplorer.org

- coverguyPA1.jpg (23.42 KiB) Viewed 4742 times
And while searching the PA through seomoz plugin I found the following:

- coverguyPA.jpg (25.5 KiB) Viewed 4742 times
As you can notice there is a difference in both metrics.
Also kindly see the image below which shows PA of a site toronto-internet-marketing.ca, if you analyze how they are building their backlink; its very very clear that they are using
spammy back link techniques.

- spam-site-pa.jpg (27.33 KiB) Viewed 4742 times
How can OSE give PA of 60 for these kind of sites. The funny part is that the site is ranking in Google for many of the keywords which is used as Anchor text (90% Spammy Techniques), Again Making Google fool, Don't know how all these Panda & EMD updation are effecting in SERP. Its seems the result is opposite of whatever Google says & updates in Panda & EMD.
Coming to PA, PA is a good factor for understanding a site authority in search engines but believing it completely is not fair.
what Seomoz says about PA?
"
Seomoz score Page Authority on a 100-point, logarithmic scale. Thus, it's easier to grow your score from 20 to 30 than it would be to grow from 70 to 80. We constantly update the algorithm used to calculate Page Authority, so you may see your score fluctuate from time to time."
Technical Definition of Page AuthorityIt is based off of the
Linkscape web index and includes link counts, mozRank, mozTrust, and dozens more. It uses a machine learning model to predicatively find an algorithm that best correlates with rankings across thousands of search results that we predict against.
So these algorithms calculate PA based on the backlinks of the site. if we can manipulate backlinks, we can definitely manipulate seomoz algorithmic calculation.
Thanks!!