Nielsen, the television research firm, acknowledged Friday that it had been reporting inaccurate ratings for the broadcast networks for the past seven months, a mistake that raises questions about the company’s increasingly criticized system for measuring TV audiences.

The error wound up benefiting one network, ABC, while negatively affecting the others, according to people briefed on the problem. In a telephone call with reporters, Nielsen executives would not confirm that it had resulted in added viewers for ABC, saying they could not discuss individual clients.

An ABC executive confirmed that the glitch had improved the network’s ratings. As for Nielsen, its executives played down the discrepancy in viewing totals, saying they fell between 0.1 percent and 0.25 percent of the viewing totals.

But it remained unclear how the mistake would affect the billions in advertising dollars based on Nielsen’s ratings, as well as the company’s reputation. And several television and advertising executives expressed degrees of anger and incredulity at both the incorrect ratings and the amount of time — seven months — it had taken to discover the problem.

“These ratings are the currency of the business,” said Alan Wurtzel, who heads research at NBC. “Any time that currency is under suspicion, it’s a concern.”

Nielsen has long reigned as the main source that the entertainment industry uses to measure TV audiences, and its ratings are the currency on which nearly $70 billion in advertising dollars are traded each year in the United States.

Nielsen executives Pat McDonough and Steve Hasker said repeatedly in their news conference Friday that the incorrect ratings fell “well within the tolerance of statistical error.” They said any changes in numbers or the rankings of programs would be largely insignificant and would be corrected when Nielsen issues new ratings on Monday.

But in a statement sent to clients, the company said, “In the vast majority of cases the impact is small, but in a handful of cases the impact is more material.”

Wurtzel said NBC would press Nielsen for more information. “I’m asking for it,” he said. “How do we ever begin to do any kind of tracking or historical analysis if you can’t get accurate data?”