It seems that there was a misprint in the thousands of TV watching diaries it sent out for the May Sweeps period. The diaries are the sole data source for more than 150 local TV markets in the U.S. The error affected how people could fill out the diaries. There were also reports that issues with Nielsen's call center operations (used for recruitment and follow-up contact) that stations fear may also have impacted the ratings numbers.Concerns were strong enough that station reps complained to the Media Rating Council (MRC), which discussed the issue with Nielsen reps at a recent meeting.
Nielsen, for its part, claimed that adjustments were made in a timely fashion to procedures and the data that the results are reliable - although the refused to indicate the extent of the problems or how they "fixed" their results.
“Nielsen ensures that any data provided to clients meet or exceed the requirements set by the industry for representative results,” the company said in a statement.Basically, Nielsen's saying "trust us," despite the fact that the MRC revoked their accreditation for the Nielsen diary-only ratings service two years ago, and acknowledgement of multiple problems and errors in the diary-only ratings system in recent years. They also recently turned down an appeal for re-accreditation, in essence saying that Nielsen has not "met or exceeded industry standards." Nielsen's claim is not likely to put many of the concerns of the local TV industry about current audience measurement systems to rest.
Source - Nielsen Diary Misprints Impact Sweeps Results, MediaDailyNews