There are plenty of video ads being sold and served up on the Web and mobile devices — but close to half of all video ads that don’t run on YouTube are never seen by a user, according to a Google study.

Of course the report, released by Google this week, is self-serving for the Internet giant to make the case that YouTube is far better than other video sites in this regard: The average viewability of video ads across the Web (not including YouTube) is 54% whereas on YouTube it’s 91%, according to the company.

Google’s study analyzed data from its video-advertising platforms, including Google, DoubleClick and YouTube, for the month of April. The standard for what counts as a “viewable” ad is that at least 50% of the ad’s pixels must be visible onscreen for at least two consecutive seconds, per Media Rating Council and IAB guidelines.

Why were so many ads simply not viewed? Google found that of non-viewable ads, 76% were in a background tab or never on screen at all. The remaining 24% were scrolled off-screen or abandoned in fewer than two seconds.

According to Google, larger video players significantly boost the chances an ad will be seen. Devices make a difference as well: For non-YouTube sites, ad viewability on mobile was 83% and on tablets was 81% versus 53% on desktop computers.