Time Warner Cable and several prominent cable networks will not take part, at least initially, in Nielsen Media Research's effort to measure the average viewership for all the national commercial minutes that run during a program, according to a report in The Wall Street Journal.

NBC Universal said in a statement Monday its cable networks would opt out of Nielsen's plan. Joining NBC Universal and TWC in their refusal were MTV Networks, Discovery Networks and ESPN.

The refuseniks are in accord with the Cabletelevision Advertising Bureau, which has recommended that its members not take part for now.

Cable executives object that Nielsen systems have trouble distinguishing between a national and a local ad within the same commercial break on a cable channel.

Distribution of cable programming isn't nearly as uniform as it is on broadcast television, and Nielsen thus can't track commercial minutes on their outlets as well as it does on broadcast television, the WSJ reports.

Nielsen expects to unveil the commercial-ratings data in mid-December, having postponed the start date by a month to continue discussing the system with TV networks.