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The instances when a page view, as measured by our website stats, does not equate to Google's impressions are described in the FAQ thus:
We count impressions only when our ad code is executed by a user's browser. There are therefore several factors that would cause a discrepancy between the impressions you register and the impressions listed in your AdSense account. Instances that wouldn't execute the code include:
* Other spiders, robots, crawlers from other search engines
* Browsers that don't support the <iframe> tag
* Programs that people may have written to grab website
Hope this clears up any misunderstandings.
This would suggest also that PSA ads would count as one impression, so CTR rate can also be affected by the proportion of your pages carrying PSA ads.
I have confirmed this. I removed AdSense code on all pages on a specific site except a single page. The page receives over 1,000 page views per day and even though the page has shown nothing but unpaid PSA ads the last 4 days the number of impressions in the AdSense reports have nearly matched the page views according to the server's Apache web logs. Also the CTR has been 0.0% (no clicks) even though it had historically been significant so I've ruled out paid ads showing for others or other geographic reasons.
For example, if a page is viewed it counts as a single impression. If a user right-clicks on an ad, then right-clicks on another ad that should count as 2 clicks (whether Google throws out one is a different story).
2 clicks / 1 impression = 200% CTR.
I doubt a significant percentage of users right-click ads and I doubt a significant percentage of those who do click multiple ads without a page load, but I thought it was worth pointing out.