Months of Nielsen television ratings for national broadcast networks could be wrong, the company said Friday.
"In response to recent ratings irregularities, Nielsen conducted an extensive internal investigation of our systems and processes," a press release said. "On Oct. 6, 2014, we uncovered a technical error that impacts national network television ratings over several months."
The company said this error did not affect cable networks or local television stations.
Nielsen said it will reprocess all of the impacted data going back to Aug. 18.