ComScore raised eyebrows with research last year showing 31% of online display ads are never actually viewed, but upon further review, things are even worse: its latest data indicate 46% of ads are never seen by website visitors.

The latest data comes after more than a year of additional tracking by the company's Validated Campaign Essentials service, said comScore Chairman Gian Fulgoni in a presentation at the Advertising Research Foundation Audience Measurement 8.0 conference in New York on Monday.

ComScore counts 22 of the top 25 U.S. advertisers as VCE clients, including Procter & Gamble Co. and Kellogg Co. But the new data, suggesting that fewer people see online ads than previously thought, likely owe to the fact the service has branched out beyond premium publishers and blue-chip advertisers in the past year, Mr. Fulgoni said. At many "lower tier" sites, in-view rates are well under 50%, he said.

Experience at Kellogg shows viewability matters a lot: a 40% improvement in ad viewability produced a 75% increase in sales life from digital advertising, said Aaron Fetters, director of Kellogg's Insights and Analytics Solutions Center.

The ability to get real-time ad viewability data, which began last year, "was like turning on a light in a dark room," Mr. Fetters said. Working with Starcom MediaVest Group to stop buying and paying for the "roughly half" of digital ads never seen by people has been part of a system that's had dramatic impact on the return on investment for Kellogg brands, he said.

Now in year two of working with VCE, he said Kellogg has produced fivefold and sixfold increases in digital advertising ROI for two of its brands. Part of that owes to Kellogg increasingly using programmatic trading via its in-house trade desk, but it's partly the result of better viewability and audience tracking.

"If you can't be viewable," he said, "then there's no point worrying about anything else."

It's not certain why Kellogg got an even bigger bump in sales lift than it got in viewability. But Mr. Fulgoni said, "It could be that you have more of your campaign on premium content sites" when eliminating non-viewable ads.

Certainly that was a view favored by Scott McDonald, senior VP-research and insights with Conde Nast. Five years ago, he noted that he collaborated with CBS Chief Research Officer David Poltrack on a study released at the ARF Audience Measurement conference showing 15% of digital ads weren't viewable. The research was criticized at the time, Mr. McDonald said, but now appears to be a conservative estimate.

ComScore counts an online ad as viewable if at least 50% of pixels are in view for at least half a second for a site visitor. That may not be the strictest standard, but it's the minimum favored by the industry's Making Measurement Make Sense (3MS) initiative and the Media Rating Council.