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Howdy folks,

Every time we do an index update here at Moz, we get a tremendous number of questions about Domain Authority (DA) and Page Authority (PA) scores fluctuating. Typically, each index (which release approximately monthly), many billions of sites will see their scores go up, while others will go down. If your score has gone up or down, there are many potential influencing factors:

1. You've earned relatively more or less links over the course of the last 30-90 days.
   Remember that, because Mozscape indices take 3-4 weeks to process, the data collected in an index is between ~21-90 days old. Even on the day of release, the newest link data you'll see was crawled ~21 days ago, and can go as far back as 90 days (the oldest crawlsets we include in processing). If you've done very recent link growth (or shrinkage) that won't be seen by our index until we've crawled and processed the next index.
2. You've earned more links, but the highest authority sites have grown their link profile even more
   Since Domain and Page Authority are on a 100-page scale, the very top of that represents the most link-rich sites and pages, and nearly every index, it's harder and harder to get these high scores and sites, on average, that aren't growing their link profiles substantively will see PA/DA drops. This is because of the scaling process - if Facebook.com (currently with a DA of 100) grows its link profile massively, that becomes the new DA 100, and it will be harder for other sites that aren't growing quality links as fast to get from 99 to 100 or even from 89 to 90. This is true across the scale of DA/PA, and makes it critical to measure a site's DA and a page's PA against the competition, not just trended against itself. You could earn loads of great links, and still see a DA drop due to these scaling types of features. Always compare against similar sites and pages to get the best sense of relative performance, since DA/PA are relative, not absolute scores.
3. The links you've earned are from places that we haven't seen correlate well with higher Google rankings
   PA/DA are created using a machine-learning algorithm whose training set is search results in Google. Over time, as Google gets pickier about which types of links it counts, and as Mozscape picks up on those changes, PA/DA scores will change to reflect it. Thus, lots of low quality links or links from domains that don't seem to influence Google's rankings are likely to not have a positive effect on PA/DA. On the flip side, you could do no link growth whatsoever and see rising PA/DA scores if the links from the sites/pages you already have appear to be growing in importance in influencing Google's rankings.
4. We've done a better or worse job crawling sites/pages that have links to you (or don't)
   Moz is constantly working to improve the shape of our index - choosing which pages to crawl and which to ignore. Our goal is to build the most "Google-shaped" index we can, representative of what Google keeps in their main index and counts as valuable/important links that influence rankings. We make tweaks aimed at this goal each index cycle, but not always perfectly (you can see that in 2015, we crawled a ton more domains, but found that many of those were, in fact, low quality and not valuable, thus we stopped). Moz's crawlers can crawl the web extremely fast and efficiently, but our processing time prevents us from building as large an index as we'd like and as large as our competitors (you will see more links represented in both Ahrefs and Majestic, two competitors to Mozscape that I recommend). Moz calculates valuable metrics that these others do not (like PA/DA, MozRank, MozTrust, Spam Score, etc), but these metrics require hundreds of hours of processing and that time scales linearly with the size of the index, which means we have to stay smaller in order to calculate them. Long term, we are building a new indexing system that can process in real time and scale much larger, but this is a massive undertaking and is still a long time away. In the meantime, as our crawl shape changes to imitate Google, we may miss links that point to a site or page, and/or overindex a section of the web that points to sites/pages, causing fluctuations in link metrics. If you'd like to insure that a URL will be crawled, you can visit that page with the Mozbar or search for it in OSE, and during the next index cycle (or, possibly 2 index cycles depending on where we are in the process), we'll crawl that page and include it. We've found this does not bias our index since these requests represent tiny fractions of a percent of the overall index (<0.1% in total).

My strongest suggestion if you ever have the concern/question "Why did my PA/DA drop?!" is to always compare against a set of competing sites/pages. If most of your competitors fell as well, it's more likely related to relative scaling or crawl biasing issues, not to anything you've done. Remember that DA/PA are relative metrics, not absolute! That means you can be improving links and rankings and STILL see a falling DA score, but, due to how DA is scaled, the score in aggregate may be better predictive of Google's rankings.

You can also pay attention to our coverage of Google metrics, which we report with each index, and to our correlations with rankings metrics. If these fall, it means Mozscape has gotten less Google-shaped and less representative of what influences rankings. If they rise, it means Mozscape has gotten better. Obviously, our goal is to consistently improve, but we can't be sure that every variation we attempt will have universally positive impacts until we measure them.

Thanks for reading through, and if you have any questions, please leave them for us below. I'll do my best to follow up quickly.

No, that's not correct at all. As you can see from the other folks replying in the thread, and from reading my post and responses, we're simply saying that DA is a relative measure, not an absolute one. It's like ranking websites based on their visits rather than showing their raw number of visits. You could grow your site traffic by many thousands of new visitors, and still have a lower "rank" because others grew their traffic even more. That's just how relative metrics work.

Thanks for the feedback Joseph - I appreciate your transparency and can totally empathize with the frustration.

I think the key here, unfortunately, is in understanding and effectively explaining how the metrics of DA and PA operate and why they're not like standard counts that always go up as things get better. Clearly, we need to do a better job of that.

A good metaphor might be how rankings work for countries in various categories. For example, if Japan is ranked as having the world's best healthcare in 2015, and they improve the quality of their healthcare in 2016, are they guaranteed to still be #1?

Not necessarily.

Maybe the #2 ranked country improved even more and now Japan has fallen from #1 to #2 despite actually improving on their healthcare quality. Maybe countries 2-10 all improved dramatically and Japan's now fallen to #11 even though they technically got better, not worse.

PA and DA work in a similar fashion. Since they're scaled on a 100-point system, after each update, the recalculations mean that PA/DA for a given page/site could go down even if that page/site has improved their link quantity and quality. Such is the nature of a relative, scaled system. This is why I encourage folks strongly to watch not just PA/DA for their own pages/sites, but for a variety of competitors and sites in similar niches to see whether you're losing or gaining ground broadly in your field.

Hope that's helpful and wish you all the best.

Hi Joe - yes, it's most likely the fluctuations are because you're in that lower range. Remember that Moz can't see links you've disavowed in Google Webmaster Tools/Search Console, so we wouldn't be lowering DA/PA based on those (although, it's possible that, over time, as Google stopped counting links like that, our algorithm for PA/DA would "learn" and evolve, but that would take a while).

Hi Donna - yes, that fluctuation should be much larger on average in the tail of the web (sites with DA 0-40) than in the middle or head. This makes sense because with a relative metric, all of the factors I describe above are going to be magnified in the tail, particularly because Google's rankings change so much there and because just a few links can have such a huge impact. For metrics-savvy clients, they should be best poised to understand that since DA/PA are exponential, a few links here or there and a few valuation shifts on those links can have big swings in the 10-40 point ranges of DA/PA, whereas in the 50/60+ ranges, small shifts in link discovery or in link valuation (from us or Google) won't have as much change.

As far as a ceiling - no, we don't have a recommendation there. The idea is that as DA/PA fluctuate, especially as they get more accurate in predicting rankings (correlations & coverage), the fluctuations are generally happening because Google's changing and we're getting closer to tracking how and where (with exceptions I noted above around issues with our crawl/indexing). My biggest recommendation is to keep track of similar-sized competitors (and larger/smaller ones) so you've got a set of benchmarks for comparison.

Thanks for the note and apologies for the frustrations this causes.

Always happy to help, and especially to provide transparency. Thanks for the kind response

Hi Deacyde - that shouldn't be the message you're taking away from this post at all! As I noted above, you could improve the SEO, improve your link profile, and still see a reduction in Domain Authority as as a score due to how DA is done (on a relative scale, not an absolute scale). We could find more and better links to your site, and you'd still see a lower DA.

If a number of smaller sites in your field have all seen lower DA score in this index, that's indicative that it's nothing you've done, but rather, an indication that DA has been shifting across the board. If you're seeing rankings stay high and organic search traffic stay high, then there's nothing to worry about, and DA should still work just as well as a relative metric across sites (actually, slightly better given the improved correlations).