Hi Trevor!
As it turns out, we do have a cache on our index to help stop valid links from falling out of our view. However, we're not able to guarantee that all links we've previously indexed will be re-indexed within that cache period. So, it can certainly still happen that links will drop out of our index. In general, this helps to promote a stable index, while still preserving freshness and relevance of links that we report on.
As far as metrics like DA and PA go, it's always important to remember that these are relative metrics, rather than absolute. While it always feels good to see them climb higher and higher, a consistent score or even a drop doesn't necessarily mean that there has been no improvement. I'd definitely recommend checking out this post by Rand on ways to consider these metrics: https://moz.com/community/q/da-pa-fluctuations-how-to-interpret-apply-understand-these-ml-based-scores


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