Pat McDonough, a senior vice president at Nielsen, said this week that the company was watching youth viewing behavior “very closely,” and acknowledged in an interview that she had seen “a little bit” of a drop-off in youth viewership. She cautioned, however, that there had been fluctuations in the past, and noted that traditional television viewership in 2010 was very high. (This is possibly a reflection of economic circumstances, since television viewing tends to grow when the American economy does not.) The shorter National Football League preseason in 2011 may have also influenced year-over-year comparisons, she said.

As behaviors shift, there is likely to be a scramble to identify winners and losers. Viacom, the owner of Nickelodeon, criticized Nielsen last fall after ratings showed that the channel suffered from a sudden drop in children’s viewing.

According to data for the first nine months of 2011, children spent as much time in front of the television set as they did in 2010, and in some cases spent more. But the proportion of live viewing is shrinking while time-shifted viewing is expanding.

Zach Dulli, a director of operations for the National Council for Geographic Education in Washington, has noticed that his children, Max and Huck, like the TV set, but they like laptops and cellphones more. Now that Huck has mastered the finger swipe to turn on an iPad, Mr. Dulli and his wife, Stephanie, prepare “Baby Einstein” for him to watch on the device. Huck is eight months old.

“To us, TV is separate from the other media we use,” Mr. Dulli said. “To my sons, it’s not.”

To a child, television shows on the iPad are still television, but to Nielsen, it’s not: the company counts computer and mobile streams of shows separately, making it difficult for the television industry to get a handle on changing habits.

On Sunday, for the first time, the Super Bowl was broadcast online as well as on television, but the ads were sold separately and the ratings were reported separately. About 2.1 million unique users watched the live stream at some point during the game, while about 111.3 million people watched on television at any given time during the game. NBC says there was some overlap, but that it was hard to know how much. But the network has said that the game was the “most-watched single-game sports event ever online.”