Nielsen began including out of home viewers in its national ratings in September of 2020. But earlier this week, the TV ratings provider shared that some of those numbers may be inaccurate.

Nielsen has told clients it has undercounted some out of home viewers for that entire time, an issue they say is due to their software. 

"As part of routine testing and quality controls, we recently identified an error that caused an understatement of reported out-of-home audiences for our national TV service," Nielsen said in a statement. "While there is no impact to most telecasts, and no impact to local television, we did find some variances for events that tend to yield larger out-of-home audiences, such as live sporting events. The error has been corrected and Nielsen will be reissuing data from September 2020 to present in order to provide the industry with the most complete data."

Out of town viewing would include things like watching a given program outside of your household at a public space. It is a big problem for sports in particular, as many games are watched at parties or in a sports bar setting.

In September, the Media Rating Council suspended Nielsen's accreditation for national TV ratings. Complaints were made by clients about the company letting its national panel deteriorate during the pandemic, leading to potential undercounting of TV audiences. 

The Video Advertising Bureau said in September it would form a task force looking at measurement innovations.

"Today's announcement by Nielsen of more systemic errors that have further undercounted TV ratings for the last sixteen months is unfathomable, both for Nielsen and the buyers and sellers that use Nielsen data as trading currency," Video Advertising Bureau president and CEO Sean Cunningham said in a statement. "While the VAB and our Measurement Innovation Task Force have been looking to build back confidence in Nielsen's core measurement capabilities, the timing of this latest and stunning omission of OOH viewership not being counted across the growing broadband only home universe coincides with our recent discovery of significant defects in Nielsen's overall plans to include broadband only homes in early 2022 for local TV."