Nielsen revealed today that a “technical error” in its ratings system impacted national TV ratings over a period of several months. The company went public about the problem in a press release issued today, saying that the problem has been corrected.

The problem dates back to March 2, 2014, but was not noticed until unusual patterns were seen in connection with fall premiere week, the company said. Cable networks and local TV ratings were not affected by the glitch, the company said, but “small amounts of viewing for some national broadcast networks and syndicators were misattributed.”

A software fix was deployed Thursday that corrected the problem, the company added.

“All previously released data since September 22nd will be reprocessed and reissued by Oct. 17, 2014. We will also reprocess all of the impacted data going back to Aug. 18, 2014, when the first new season broadcast network program aired. This data will be reissued by Oct. 31, 2014,” Nielsen said.

The company is conducting an impact analysis to decide whether additional weeks of data will need to be reprocessed. But according to Nielsen, the impact in most cases is relatively minor.

“This issue has to do with difficult-to-attribute content called ‘all other tuning with code’ (AOT with code),” the company said. “This data represents between 0.1% and 0.25% of all viewing minutes that we credit nationally. In the vast majority of cases, the impact is small; in a handful of cases, the impact is more significant.”

Nielsen added: “As part of our investigation, we have also determined that there are no issues with the National People Meter, our data collection process, our panel, our TV audience measurement methodology or the total TV viewership data produced during this affected period.”

Please click here to read the full statement from Nielsen.