NEW YORK (CNN/Money) - More U.S. households were tuned to cable television networks than broadcast networks last month during prime time, the first time cable has bested over-the-air competitors during a crucial sweeps period.

Ratings for the sweeps period, used by the networks to set ad rates, help determine how advertisers spend $30 billion a year to reach TV watchers. That's why cable and broadcast networks promote their most attractive programs during the sweeps period.

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Cable has beaten broadcasters in the little-noticed July sweeps period in the past, when broadcasters air reruns or alternate programming and overall viewership is lower, and it has beaten the networks in a handful of non-sweeps periods.

But this is the first time it has topped the broadcasters in any of the closely followed February, May or November ratings periods, according to an analysis of Nielsen Media Research ratings performed by the Cabletelevision Advertising Bureau (CAB), an industry group of networks and cable operators.

The analysis was also performed by the research department of Turner Broadcasting, which owns broadcast as well as cable networks. Turner and CNN/Money are both owned by Time Warner Inc. (TWX: Research, Estimates), the world's largest media company.

A Nielsen spokesman said the service was still reviewing the data, but that it believed the totals to be accurate.

The numbers from Turner's analysis show that all cable networks together reached an average of 39.1 million homes during prime time in November, while the broadcast stations reached an average 38.5 million homes. A year ago during the November sweeps, cable reached 35.9 million homes, while broadcasters reached 38.7 million.

Those numbers include cable networks that do not accept advertising, as well as public television broadcasts and scattered independent stations. They also include PAX, the distant No. 7 broadcast network.

Comparing the ad-supported cable networks to the six major broadcast networks -- CBS, NBC, ABC, Fox, WB and UPN -- the switch from broadcasters to cable during the recent sweeps is also evident, though by a more narrow margin. Even if PAX is included, the broadcast networks had less than a 50 percent share of viewership, according to the CAB.

"The fact that cable performed better during sweeps is an important sign," said Steve Raddock of the CAB. "There's more emphasis then by the broadcasters in terms of programming and promotion than any other time of the year."

A spokesman for the National Association of Broadcasters, which represents the over-the-air networks, did not return calls for comment Wednesday.

A spokesman for Viacom (VIA.B: Research, Estimates)-owned CBS, the top rated network in the November sweeps, said the network wasn't concerned about broad cable-broadcast viewership comparisons.

"CBS was the only broadcast network to enjoy increased ratings across the board," spokesman Dana McClintock said. "We could not have enjoyed a more successful sweeps."

An executive with Fox, whose parent company Fox Entertainment Group owns several cable networks, also questioned the validity of comparing total broadcast viewership to total cable viewership.

"You're talking about taking hundreds of channels adding them up to six broadcast networks," said Giles Lundberg, executive vice president for research and marketing at Fox. "You wouldn't do it in any other business, take these few companies and compare them these hundreds. Broadcast is still the only television medium that is a mass medium."

Fox saw its overall ratings down six percent in the November sweeps compared to a year earlier. But Fox said that decline can't be written off directly to cable.

"When ratings go down, sure we're concerned," said Lundberg. "There is no question that there is more competition, more cable networks, more entertainment options beyond the television set. Is there a place for both (broadcast and cable)? Certainly. Consumers are asking for choice."

The other major networks had no immediate comment.

Broadcasters held their lead with 18- to 49-year-olds, a key demographic group for advertisers, but that margin continued to narrow. The six major networks reached an average of just under 21 million viewers in that age group, down 8 percent from a year earlier, while ad supported cable reached 18.5 million, up 6 percent from November 2002.

And broadcasters lost their lead among another group wooed by advertisers, men aged 18 to 34. Ad-supported cable grabbed 4.2 million of those viewers, up 1 percent from a year earlier, while the six networks saw a 16 percent drop in that group's viewership to 3.7 million. The networks have recently been questioning whether Nielsen is accurately measuring that group's television viewership.

Of course because there are dozens of major cable networks compared to a relative handful of broadcast networks, advertisers must still turn to the broadcasters to reach the larger audiences in one bite. And most of the major broadcast networks have corporate parents in common with some of the top-viewed cable networks.

But these numbers could help shift ad dollars to the cable networks.

Media analyst Jack Myers projects that advertisers will spend $16.5 billion on broadcast network ads this year, up 3 percent, while they'll spend $13.5 billion on cable advertising, up 8 percent.