"With the advancement of new technologies we now know that many display ads that are served never actually have the opportunity to be seen by a user," said Google group product manager Sanaz Ahari in a blog post.
Those ads appear outside the viewable area of a browser window. Once you factor in bots, even fewer ads are seen by the people advertisers are paying to reach.
The report comes at an uneasy time in the digital ad world, when fraud and poor inventory quality are raising doubts about the industry's ability to deliver on what is sold to advertisers.
The publication of data from Google, the world's largest ad-tech company, is likely to increase advertiser concerns about the trustworthiness of the digital ad industry supply chain. Google's finding is based on information collected from its Doubleclick ad server and its display network in October 2014.
Google also found a smaller number of publishers with significantly greater numbers of unviewable ads, which skewed the numbers higher. The average publisher viewability is 50.2%, according to Google.
The report detailed a number of other findings about viewability based on data collected in July 2014. Among them: vertical ad units are the most viewable and placing ads above the fold does not guarantee viewability. Worth noting, Google does offer a viewability product called Active View, which may explain its decision to release the report to the public.