In an unprecedented move, ratings giant Nielsen has acknowledged problems with its audience estimates in recent months for Los Angeles radio stations.
Earlier this week, Nielsen delayed the release of its May radio ratings, saying it needed more time to perform "quality control reviews" because of concerns that its audience estimates might have been compromised.
On Wednesday, Nielsen acknowledged that the problems were probably more extensive. Nielsen said it would revise the ratings issued for April because those numbers also appeared to be tainted.
"After reviewing preliminary data for the Nielsen Audio Los Angeles PPM market, we discovered inconsistencies with the household ratings that lead us to conduct a more in-depth analysis of the data," Nielsen said in a statement distributed to radio stations on Wednesday.
Nielsen had planned to release its May ratings for the Los Angeles market on Monday. Instead, the company alerted radio stations that it needed more time to comb through its data to make sure the ratings were legitimate.
Nielsen has been attempting to verify that individuals who participated in its sample audience panel were truly independent and did not have ties to any radio stations or radio personalities that were being measured.
The ratings agency said Wednesday that it needed to "remove a household from the panel for not meeting our quality standards."
Radio industry insiders said such actions were unprecedented. One person said the move marked the first time in more than a decade that the release of radio ratings in Los Angeles had been postponed due to such concerns.
Another industry executive said measurement glitches occasionally occur in small markets, but not in Los Angeles, the nation's second-largest media market, where Nielsen uses more advanced technology to gauge audience levels.
In Los Angeles and other large markets Nielsen uses a device called a personal people meter (PPM), which it provides to members of its sample audience. The meter electronically records programs that panel members listen to.
Nielsen said it would release the May ratings on Tuesday. Revised ratings for April, which were released last month, are expected to be completed and distributed to stations at the end of next week.
"Nielsen is committed to upholding the highest standards of data integrity and acts swiftly to meet those standards," the company said in a statement. It called the audience pool problem in Los Angeles an "isolated situation."
The accuracy of Nielsen's estimates is critically important to radio stations, which rely on the firm's ratings to set prices for their commercial time.
Los Angeles is considered the nation's largest radio market, in terms of revenue. An estimated $1 billion is spent on radio advertising each year, including digital platforms, according to industry sources.
Problems with Nielsen's sample audience date back to last summer, when the firm launched an investigation into possible audience pool tampering.
The Nielsen investigation came to light last fall, after Spanish-language radio host Ricardo Sanchez, who then worked at Spanish Broadcasting System's popular La Raza station KLAX-FM (97.9), rocketed to the top of the ratings for morning drive-time programs in the spring of 2013.
Executives at rival stations suspected problems with the ratings, and they complained to Nielsen.
The issue is a thorny one for Nielsen, which acquired the Arbitron radio ratings company last year, because the situation exposed potential problems with its results and perhaps a reliance on too small of an listener pool to accurately extrapolate audience levels for such a large and ethnically diverse region as L.A.
Last fall Sanchez was abruptly taken off the air by Spanish Broadcasting System. At the time, Sanchez was involved in tense contract negotiations with the station. He was told to report for work and sit at a desk. Sanchez has said he did nothing wrong.
The problems with the Nielsen audience panel appear to have continued long after Sanchez left the La Raza station. Sanchez, who goes by the nickname "El Mandril," returned to the airwaves earlier this year -- but at a new station, KXOS-FM (93.9).