With the industry moving toward paying for viewable ads rather than served impressions, advertisers want to know which ads are getting viewed while publishers are trying to enhance viewability.

New data from Google helps paint a clearer picture of where those views are coming from. The tech co today released new research from its viewable impression measurement tool, Active View.

According to the report, a small number of publishers are serving most of the non-viewable impressions. In Google’s study of its display advertising platforms DoubleClick for Publishers, DoubleClick Ad Exchange and the Google Display Network, overall 56.1% of impressions it measured are not seen, but on average 50% of ads are viewable on publishers’ domains, which indicates a few publishers serving a large number of non-viewable impressions is bringing down the average.

A display ad is considered viewable when 50% of its pixels are in view for a minimum of one second, according to Media Rating Council (MRC) guidelines. (Google Active View is certified by the MRC.)

The study also lays out the most viewable positions for ads and confirms that when it comes to display, size does matter, with a vertical ad being preferred to horizontal. And viewability varies depending on the content of a site, being highest on reference sites (51.9% according to Active View data) and lowest on news or real estate sites (38.8 and 38.7% respectively).

The data was  collected during the months of July and October. The results are based on display ads in browsers but not mobile in-app or video ads.

Check out the infographic for a breakdown of the findings.