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Viewability is a hot topic for marketers right now: Nobody wants to pay for an ad that's never actually seen by a human.

There's a number of reasons why ads are deemed not "viewable," such as users never scrolling to the part of the page where the ad appears, ads loading in a background, or users simply scrolling past the ad too quickly to count as an impression (the industry standard for a viewable video impression is that 50% of an ad's pixels are visible on the screen for two consecutive seconds or more.)

https://www.thinkwithgoogle.com/infographics/5-factors-of-video-viewability.html