Time Warner Inc.’s Turner cable networks and Walt Disney Co’.s ESPN have joined the ranks of cable networks that won’t participate in a new commercial-ratings system from Nielsen Media Research, which aims to measure the average viewership for all the national commercial minutes that run during a program, the Wall Street Journal reports.

General Electric Co.’s NBC Universal said Monday its cable networks, which include USA, Bravo and Sci-Fi, wouldn’t participate. (See related article.)

The Cabletelevision Advertising Bureau also is advising its members to not take part in the system until Nielsen refines some of its methodology, said Sean Cunningham, chief executive and president of the trade association.

At issue, cable executives say, is whether Nielsen can measure ads on cable effectively. Cable executives have pointed out in recent weeks that Nielsen systems have trouble, for example, differentiating between a national and local ad within the same commercial break on a cable channel. Nielsen has said it expects to improve its methodology.