Executives at the research firm Nielsen admitted last week that their ratings system for network television programs was not working correctly the last seven months.

The foul-up may have resulted in certain shows receiving higher ratings than were deserved because some viewer numbers were “misattributed,” or given to the wrong program. Nielsen officials say the mistakes were caused by a programming error.

For network executives, the news was both shocking and unsettling, given that the ratings system can make or break a show—or a vice president’s career.

“This is the equivalent of an earthquake. When you can’t trust the ground you’re standing on, it’s disorienting,” a network insider told CNN. “What else has gone wrong that we don’t know about?”

The malfunction appeared to benefit ABC programming at the expense of that of Fox, CBS and NBC. “These ratings are the currency of the business,” Alan Wurtzel, who heads research at NBC, told The New York Times. “Any time that currency is under suspicion it’s a concern.”

Advertisers are concerned about the length of time it took Nielsen to find the problem. “The big concern on our part is that this happened, and it has happened for months, and nobody noticed,” Kate Sirkin, executive vice president of global research at the Publicis Groupe’s Starcom MediaVest Group, told the Times. “That is scary because we pay millions of dollars for Nielsen to do this complicated thing, but that is what their job is.”

-Noel Brinkerhoff, Steve Straehley

To Learn More:

Billions of Ad Dollars at Stake in Nielsen Ratings Glitch (By Frank Pallotta, CNN)

TV Ratings by Nielsen Had Errors for Months (by Bill Carter and Emily Steel, New York Times)

Nielsen: Technical Error Causes Months of False TV Ratings (by Brian Steinberg, Variety)