In the continuing war between the broadcast networks and Nielsen Media Research, CBS fired another salvo today, this time about problems in late night ratings numbers. The network claims that during the last three major TV sweeps periods, November, February and May, there is a major disparity in 18-to-49 age demographic ratings between Nielsen's National Television Index and National Station Index for "The Late Show With David Letterman." The problem, CBS said, is that the NSI numbers, which measure local markets, are much higher than the NTI numbers, which measure viewing nationwide. CBS said the potential ad revenue disparity could reach $40 million. Nielsen responded by saying that both measurements, said because of different methodologies, both measurements could be correct. Responded David Poltrack, exec VP-research and planning for CBS, "That's absurd."