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Hi I was just looking at a competitors new site in OSE and it had a DA and PA of 1 which I would expect and also no links, which again I would expect, basically the guy used to design stuff for me and thinks he can do a better job so he is giving it ago. But he has a spam score of 9/17 but no links - how is this even possible. Thanks Andy

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: 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. 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. 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. 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.