TV ratings company Nielsen admitted Friday that a technical error produced inaccurate measurements dating back to March 2014.
The company told broadcasters it discovered the issue on Monday and deployed a software fix Thursday. The error affected 0.1% to 0.25% of viewing minutes for some national broadcast networks and did not impact cable network and local TV ratings.
“In the vast majority of cases the impact is small, in a handful of cases the impact is more material,” the company said in a statement.
Nielsen said it will reprocess impacted data going back to August 18, the start of the fall TV season, and is assessing whether to reprocess additional weeks.
For years, the ratings company has been a target of criticism for using a small sample of about 5,000 households to reflect the TV habits of the rest of the country. Broadcasters are largely beholden to Nielsen ratings, using them to determine how much to charge advertisers and whether to cancel or renew a program.