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It seems likely that the interstitial is at least contributing to this pattern given the correlation during your test and given the fact that Google has specifically noted that they view interstitial pop-ups (at least certain implementations) as detrimental to the user experience. It is also possible that it isn't the interstitial itself that is raising a red flag as a ranking signal, but some other related issue (for instance, if the interstitial causes a significant page load delay, higher bounce rate, or if there is some rendering issue caused by the implementation of the pop-up). As you note, if the interstitial is the issue, I would have expected it to affect mobile primarily and desktop less so - so the fact that the result is the opposite is surprising and may indicate a different cause for the traffic dips. I assume that you have compared this data not just WoW but YoY and are certain that the pattern isn't seasonal. We do know, however, that Google is starting to move in the direction of mobile first in how they rank sites, making the mobile index and experience their primary source for rankings of both mobile and desktop versions. In any case I would recommend testing a version of your pop-up which is not of a type that Google would consider "intrusive" (see this post for more detail: http://searchengineland.com/google-confirms-rolling-mobile-intrusive-interstitials-penalty-yesterday-267408) or adding a trigger to it (so for instance, don't show the popup immediately but only if the user scrolls a certain percentage of the page, or remains on the page for a set period of time).

Hi Viswanatha, Short answer, no. Page Authority is based mostly on link metrics. That said, it takes many factors into consideration, including Domain Authority and trust metrics. Page Authority is a comparative metric, meaning that all other factors being equal, the page with the higher PA would have a better chance of ranking for it's particular keyword. So 2 pages, each optimized equally for the same keyword, with exactly the same title tag, etc (forget for a moment that this is duplicate content) the page with the higher PA would likely rank higher than the lower one. You can read more about PA here. http://www.seomoz.org/learn-seo/page-authority