Kraft Foods, Greyhound Lines and Capital One Financial have bought some strange ads on the Internet lately. What's so strange about them is that they're invisible.
The companies might not have known about their invisible display ads—the kind that are supposed to appear alongside content on Web pages—if not for Ben Edelman, an assistant professor at Harvard Business School who studies Internet advertising.
Mr. Edelman says his research shows that all three marketers, and many others, have fallen victim to Web sites that use such ads as a way to sell more ad space than they have.
The Web sites can get away with it, he says, because online advertisers don't always audit their campaigns for proof their ads are appearing. It isn't clear how common these ads are or how much they cost marketers.
Verifying that ads appear is an issue that has long plagued traditional media, particularly commercials on local TV stations. But a single online ad campaign can appear on thousands of Web sites, making verification even harder.
Advertisers often buy display ads based on the number of times they are loaded onto a page, rather than the number of clicks they get. Over the past, year, an increasing number of scams have sought to take advantage of that pricing system as advertisers have started buying more of their online ads via middlemen called ad networks, instead of directly from the Web sites themselves. These networks sell ad space at cheap rates across thousands of sites, and they don't always weed out illegitimate players.