Google found that not all ad impressions are viewed. During research conducted on the view-ability of ads there were a number of important characteristics discovered regarding ad impressions. They released the information in a survey results report in November 2014. Here’s a link to the report: The Importance of Being Seen.
There were some obvious behaviors confirmed such as if an ad impression is below the fold it is less likely to be viewed. But there were also some more subtle things like being at the top of the page isn’t as good as just above the fold.
They also defined what constitutes an ad impression according to Media Rating Council and IAB standards. If you like data this is a good report to read. But this is also a good report to read if you just want a better understanding of the various factors that make an effective ad impression.