(Reuters) - Nielsen, the dominant company that provides viewership data of TV shows, said on Friday it found an error in its database that could impact several months of TV ratings.

A “technical error” was introduced on March 2, but not discovered until Oct. 6, Nielsen said in a statement. On Thursday, the company deployed software to fix the problem.

The glitch could affect the major broadcasters including Walt Disney’s ABC, Comcast’s NBC, and the national networks of CBS Corp and Twenty-First Century Fox Inc.

Cable networks and local TV stations were not impacted, Nielsen said.

While it is not clear how much the problem will swing ratings in either direction - and Nielsen said it will undertake an “exhaustive post-mortem” - it is a blow for the company that supplies data to advertisers. Ratings are the currency used to negotiate the cost of TV commercials.

Networks executives also use ratings as bragging rights to crow about how many viewers tune in, especially during September, which marks the start of the new season.

David Poltrack, chief research officer at CBS, said the network had alerted Nielsen that it might have a problem with the ratings.

“The message we have to them is why didn’t you know about this?” he said. “What have you done to make sure it doesn’t happen again, and what are you doing to make sure the next time it happens you catch it and you catch it quickly.”

Poltrack said CBS expects that when Nielsen re-issues the ratings there should be no significant impact to CBS.

An ABC spokesman said in a statement: “Our entire industry relies upon Nielsen for accuracy and veracity, and we hope that they can quickly resolve this issue. We’re confident that the momentum we’ve seen across the network so far this season will continue, including delivery of the No. 1 new drama and the No. 1 new comedy on television.”

Representatives for Fox and NBC declined to comment.

Nielsen said in a statement the error was generally imperceptible, until it saw high viewing levels associated with fall season premiere week.

“As a result, small amounts of viewing for some national broadcast networks and syndicators were misattributed,” it said.

Nielsen said it is reprocessing the data but only going back until Aug. 18. The networks must specify if they want Nielsen to do the same for the ratings preceding August.