More than half of digital display ad impressions are not actually seen by consumers, according to a recent report from Google.

The study, based on an analysis of July and October 2014 data from Google-owned advertising platforms such as DoubleClick, found that 56% of all display impressions are not viewable.

A display ad was considered viewable by the researchers when 50% of its pixels were in view on the screen for a minimum of one second. The viewability rate reflects the percentage of ads determined viewable out of the total number of ads measured.

Though the average viewability rate was just 44%, viewability for individual campaigns varied widely based on various circumstances, the analysis found. In particular, four factors were found to strongly influence ad viewability rates.

1. Publisher

Viewability is not evenly distributed across all publisher domains, the analysis found. In other words, a small number of publishers are serving most of the non-viewable impressions.

2. Page Position

The most viewable display ad position is right above the fold, not at the top of the page, the analysis found.

There is a significant drop-off in ad viewability below the fold of a page on a browser. However, not all above-the-fold impressions are viewable, and many below-the-fold impressions are viewable.

3. Ad Format and Size

The most viewable display ad sizes are vertical units since they tend to stay on screen longer as users move around a page.

4. Publisher Industry

Viewability varies significantly across content areas, with publishers in the reference, community, and gaming verticals having the highest rates.

About the research: The report was based on an analysis of July and October 2014 data from Google-owned advertising platforms such as DoubleClick.