Nielsen Media Research is at loggerheads with America's television networks over a large fall in the number of young men watching TV.

Earlier this week, the research giant sent a letter to TV executives informing them that prime-time viewing among 18 to 34-year-old males is 8% to 12% down compared with last year.

The letter relates to Nielsen figures from last month, when many networks were dismayed to see their new shows failing to attract as many advertiser-friendly young viewers as hoped. Some TV firms argued that Nielsen's data were incorrect.

The research firm has noticed a gradual decline in TV viewing among this demographic for several years, as the internet, video games and DVDs have grown in popularity. But the average decline has been around 2% per annum, far less than the drop recorded last month.

On Tuesday, after weeks of double-checking its figures, Nielsen confirmed the fall. According to the letter, the group "found no systemic errors," but added that "clients have asked us to perform a number of diagnostic analyses relating to sampling and data collection, and we expect to complete this work very soon."

Some TV executives remain unhappy with Nielsen's figures. David Poltrack, research head at Viacom-owned network CBS, called for a further review of the data. "There's nothing that suggests we have confirmed that this change in behavior is likely to be lasting as opposed to some short-term phenomenon that will right itself over time," he commented, adding that the Nielsen sample "may not be truly representative."

At rival network NBC, research president Alan Wurtzel blasted the lack of competition in the research market. "Nielsen is a monopoly," he complained. "I don't believe they have the incentive as a business to try to improve."

Nielsen, however, is sticking by its data. "When ratings are up, it's the programs," noted spokesman Jack Loftus. "When they're down, it's Nielsen."

Data sourced from: The Wall Street Journal Online; additional content by WARC staff