LOS ANGELES (AP) -- Americans who watch the most video online tend to watch less TV, according to The Nielsen Co., a finding that overturns a longstanding belief that people are watching more programming over all devices.

The ratings agency said Wednesday that starting last fall, it noticed a segment of consumers who were starting to make a trade-off between online video and regular TV. The activity was more pronounced among people ages 18-34.

The finding could be troubling to television networks that have been putting shows online in order to reach new audiences. The hope was they wouldn't diminish viewership on television, where they still make most of their advertising revenue.

Nielsen polled about 2,600 people who said they watched videos online in the first three months of the year, and divided them into fifths based on how much they watch. The fifth that watched the most video online consumed nearly 19 minutes a day, and also watched the least amount of television, at about four hours and 32 minutes a day. The fifth that watched the least online video - at less than a tenth of a minute a day - watched the most TV at 4 hours and 50 minutes.

Nielsen said such a finding doesn't indicate that people are about to drop their pay TV packages to watch video only online, a notion known as "cord cutting." About 91 percent of TV households still paid for a TV subscription in the first quarter, and most of the changes had to do with people switching between cable, telephone and satellite companies.

Overall, TV viewing crept up by 0.2 percent from a year ago to 158 hours and 47 minutes a month, while video watching on the Internet jumped 35 percent to 4 hours and 33 minutes per month. Watching on mobile devices such as smartphones rose 20 percent to 4 hours and 20 minutes a month.

Jack Wakshlag, chief research officer at Time Warner Inc.'s Turner Broadcasting System, said the decrease in TV viewing made by the heaviest watchers of online video was relatively small and "not something we view as destructive or damaging."

He noted that overall viewing went up on all platforms, meaning that Time Warner's "TV Everywhere" strategy of making content available to subscribers on multiple devices was the right one.

Nielsen also found differences based on the ethnicity of audiences.

African-Americans watched the most of any ethnic group on TV and over mobile devices, at nearly 213 hours a month on TV, and 6 hours and 30 minutes on mobile devices. Asians watched the least TV at 100 hours and 25 minutes, and the most online video at 10 hours and 19 minutes.

Hispanics were the most likely to have a smartphone, at 53 percent, followed by Asians at 48 percent, African-Americans at 39 percent and whites at 30 percent.

Nielsen's senior vice president of insights and analysis, Pat McDonough, said the study suggests that advertisers have to cast a wider net to make sure they're reaching the audiences they want.

"The real implication for advertisers is you need to think broadly," she said. "You need to think about reaching people on the screen that they're available on at that time."

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