Last week, Nielsen began informing clients that it has been understating household coverage estimates for about 20 networks since at least October 2007 because of a problem in measuring data from satellite TV distributors. This resulted in an overstatement of the ratings for those networks' coverage areas during the period.

But Nielsen has since said that national viewership ratings--which virtually all media buyers use to make deals--were unaffected. Still, some media buyers were surprised that some major networks were on the list, and wondered about other issues.

"It scares me when I see problems like this," says Gary Carr, senior vice president and director of national broadcast with TargetCast TCM, a media-buying firm in New York. "This is a small thing. But what happens when big things go wrong?"

"What is a little disconcerting is whether this is emblematic of a bigger problem that Nielsen is having," says Lyle Schwartz, managing partner and director of broadcast research and marketplace analysis for media holding company Group M. "What was the situation and how did it happen?"

Nielsen did not disclose the extent of the error--only that the problem was limited to satellite TV operators DirecTV and EchoStar, which cover around 26% of U.S. TV households, and involved the various programming packages that are sold by those companies. Nielsen said in a client notice that its process of identifying those channels in those packages needs to change: "Given the rapid growth and the increase in the number of packages and channels available, this process no longer provides the necessary level of reliability to accurately define household channel receivability, and the package templates are not consistent enough to make broad receivability decisions."

A Nielsen spokesman wouldn't comment about the specific networks on the list.

Four of the networks on the list were Comcast cable networks--E!, G4, Golf Channel and Versus. David Cassaro, president of Comcast Network advertising sales, had no comment.

Networks Whose Coverage Area Ratings Have Been Overstated Since October 2007

Biography Channel
Discovery Health
DIY
E! Entertainment
G4
Golf Channel
Great American Country
History International
Investigation Discovery
Military
MUN2
National Geographic
NFL Network
Nicktoons
Outdoor Channel
Oxygen
Science Channel
Style
Versus
VH1 Classic
WGN

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