{Updated 10.8.14, 8:35am PT to reflect that Google’s sample was of all online ads, not just theirs.)

One of the weaknesses in online advertising is whether ads are seen by actual, you know, humans. Google has now found that more than half of online ads are not on the screen for even one second.

The tech giant this week released an infographic, “Five Factors of Viewability,” showing that “many display ads that are served never actually have the opportunity to be seen by a user.” The exact figure: 56.1 percent of all ads the Google and DoubleClick display ad platforms served. Since DoubleClick’s can manage other ad networks, the finding relates to all online ads. comScore has separately estimated that 46 percent of ads are not seen.

Google uses the Media Rating Council’s definition that an ad is viewable “when 50 percent of an ad’s pixels are in view on the screen for a minimum of one second.”

Some of the reasons given for non-viewability: a user scrolled right past it, it wasn’t delivered, or it was delivered but the “viewer” was a software bot.

Below-the-fold ads, which require most users to scroll down to view, are expected to have lower viewabilities, and 60 percent of ads below-the-fold are not seen. But Google also notes that 32 percent of above-the-fold ads are not viewed, either. The most viewable position: right above the fold.

The average publisher’s visibility is 50.2 percent, Google said, so some publishers are serving more non-viewables than others for some reason.

Because of this high level of non-viewability, several efforts are underway to move from a payment system based on impressions, to one based on viewability. The Internet Advertising Bureau (IAB) has undertaken such an initiative, and Google has been rolling out its own Active View technology.

Via Quartz