A spokesman for the firm said the error was caused when a “population estimate used for a weighting variable was inadvertently transposed in a production file”. The problem was spotted during the pre-publication data validation process.

Arbitron told customers that it will now replace manual data entry with an automated system where possible, and will begin reissuing data from the beginning of the year in the first week of June.

News of the error comes as the Federal Communications Commission opens an inquiry into the system, following complaints from minority broadcasters that the ratings samples under-represent certain ethnic groups.