We updated our link metrics and Domain Authority numbers about once a month, when our upstream data provider SEOmoz does an update.
There are a number of reasons why your numbers may have changed. Here's the explanation from SEOmoz:
- The site/page received more or fewer links or more/fewer more/less powerful links. Your site's link profile may even remain completely unchanged and still see fluctuation in DA/PA because the sites pointing to you have been recalculated to have better or worse metrics.
- Google changed things in their ranking algorithm and thus our models for DA/PA, which measure and attempt to track to correlation with Google's rankings changed, too.
- The web's link graph changed, and what was "0" (the lowest possible score) is now lower/higher than before and/or what was "100" (the highest possibly score) is now higher/lower than before. Essentially, think of this as the goalposts moving because the field's gotten bigger or smaller.
- Our web index changed in size/structure as we toss our more spam/junk and crawl more/fewer webpages, potentially biasing against links we were counting or hadn't counted in prior indices.
Thus, it's very hard to know for sure whether an increase in DA/PA for a particular page is entirely tied to your efforts, Google's changes or changes to the web as a whole.