Move over Archimedes, Darwin and Einstein. When it comes to evolutionary revolution, from November 2006 nothing in the fevered world of US commercial television will ever be quite the same again.

In just four months Nielsen Media Research will for the first time publish ratings for specified commercial breaks - a media measurement breakthrough whose aftershocks will reverberate across the global advertising industry.

Despite technological advances, the basic principles of media measurement have changed little in the past sixty years, the conventional wisdom being that however many eyeballs watch a TV program, a reduced number linger into the commercial breaks.

Trouble was no-one could quantify that number, and media buyers and sellers have simply made assumptions to underpin their haggling over ad rates. Into those assumptions they factor the obvious: that viewers desert in droves during commercial breaks, to plunder the fridge or satisfy the demands of nature.

That question-mark is set to vanish with the introduction of Nielsen's new measurement methodology, with advertisers and networks agreed it will reflect a viewing drop during commercial slots.

The big unknown - the size of that drop - will be revealed come November. A substantial fall will trigger some tough negotiations, not all of which will be concerned with price. How ads are incorporated into shows will become a debating point, likely leading to demands for fewer or shorter commercial breaks.

Both camps are already digging-in. "Prices should go down," says ZenithOptimedia evp Bruce Goerlich. "If I was a buyer, I would be taking the stance of, 'Quite frankly, what you said you were delivering, you weren't.' "

Whistling in the dark, NBC Universal Television Group ceo Jeff Zucker insists there is "no reason to believe" the new methodology will lead to reduced prices. "The bottom line is that there is still no better way to reach a mass audience."

The enhanced ratings will be retroactive, recording viewing from the start of the coming fall TV season in September. Nielsen will measure average viewing numbers for all the national commercial minutes that run during a program.

However, individual commercials or specific commercial time slots won't be measured - although industry onlookers believe this is merely a matter of time.

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