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PA is the 'strength' of an individual page, whereas DA is the 'Strength' of your whole site and included a lot of factors.

To understand how to improve DA (Domain Authority) it can help to get some insight into how we gather our data, which I'll include below. Sorry in advance for the long message!

To calculate DA, we index the web by following links using a crawler. Our crawler is built on a machine-learning based model that is optimized to select pages like those that appear in our collection of Google SERPs. We feed the machine learning model with features of the URL like the backlink counts for the URL and the PLD (pay-level domains), features about the URL like its length and how many subdirectories it has, and features on the quality of the domains linking to the URL and PLD. So, it's not based on any one particular metric, but we're training the crawler to start with high-value links.

In terms of improving your Domain Authority, with the information above in mind, it's best to look at both you on-page SEO to make sure you've covered the basics.

Then you also want to look at your off-site SEO and building more links more links to your site.
There are lots of different areas of Link Building to explore, from patching up broken links, improving your internal link profile, and good ol' fashioned content creation.

Here is a great video by our founder Rand which goes over some easy link building tactics to get you started.

I hope that helps to start you off! Feel free to reach out to help@moz.com if you have any other questions,