Why so many Internet ads are never seen

More than half of all Internet ads don't capture any eyeballs, underscoring the difficulty facing both publishers and marketers in capturing the public's attention in the increasingly fractured media environment, according to data from Google (GOOG) and market researchers.

Google estimates that 56.1 percent of ads are unviewable (an ad is considered viewable if 50 percent of its pixels are visible for a second), while comScore puts it at 54 percent. A study by Vindico found 55 percent of ads can't be seen. The ramifications are huge because publishers traditionally charge advertisers for display ads on a cost-per-thousand (CPM) impressions basis.

Whether this will force ad rates down isn't clear. Some marketers may be willing to pay for a guaranteed audience, according to Tom Adams of SQAD, which tracks advertising costs.

"Viewability continues to be a big issue from an ad measurement perspective, particularly for those who are looking to standardize digital with more traditional ad measures," wrote Dan Marcec, a spokesman for eMarketer, which follows the digital ad market, in an email. "Such findings from Google reaffirm that even though there's a lot of effort and investment going into viewability ad measurement, there are still significant gains to be made in terms of actually realizing improvements. "

ZenithOptimedia noted that spending on Web-based ads overtook newspapers for the first time in 2013, and it forecasts that the Web will exceed magazines in 2015. Spending on online ads is expected to increase at an average rate of 16 percent from 2014 through 2016, according to ZenithOptimedia. Compared to spending on other media, ZenithOptimedia says the Web is "still the fastest growing medium by some distance," thanks to continued growth in search and enhancements in digital display technology and in mobile.

"That multiplies the problem," Corey Elliott, director of research at Borrell Associates, which tracks advertising, told CBS MoneyWatch. "They haven't figured out the best methods for advertising on mobile devices."

Data from Google found that the most viewable position in on a website was right above the fold and not at the top of the page. Position, though, isn't everything.

"Not all above-the-fold impressions are viewable, while many below-the-fold impressions are," according to Google.

Ads may not be viewable because they can't be loaded due to a technical glitch or because they're being "served" by bots, networks of compromised computers that hackers use. And it isn't clear how many of these ads are being viewed by such networks instead of by people.

However, a Google spokesperson wrote in an email: "I wouldn't presume it's bots -- in this case, viewability refers to whether an ad can actually be seen in the viewable portion of a page. For example, a page might load, but the person looking at it never scrolls down, so they don't see ads served at the bottom."

"[Viewability] has been a big issue in digital advertising for the past two to three years," said Andrew Lipsman, a vice president at comScore, in an interview. "The industry is slowly moving toward a viewable impressions standard."

According to the Interactive Advertising Bureau (IAB) trade group, the industry has developed two programs to address this issue: Making Measurement Make Sense and Trustworthy Accountability Group. Advertisers are increasingly paying only for the ads that can be seen.

Mike Zaneis, executive vice president and general counsel at the IAB, warns about jumping to conclusions about the data.

"We applaud Google for adding rich data to the industry's ongoing discussion around inventory quality," he wrote in an email. "However, their report does NOT suggest that 56% of all ads are called by bots and are the result of non-human traffic, in fact, I don't see any reference to fraud in their material."