Nowhere is the impact more apparent than at the CW, where recording the shows and watching them later account for nearly 17 percent of the network's viewership over a one-week period. Two years ago, it was less than 5 percent, according to Nielsen Media Research.
The time-shifting is more dramatic for individual shows. The CW even had a week where the audience of 18-to-34-year-old women for "90210" increased by a stunning 79 percent over the live broadcast.
Viewing for ABC, CBS and NBC programs are all more than 10 percent time-shifted now, too. Fox's programming is only 8 percent time-shifted this fall, in large part because it has shown postseason baseball, which very few people watch later.
"More and more people are changing the way they consume television," said Alan Wurtzel, NBC's chief research executive. "In the next few years, we will rewrite all the rules."
The most time-shifted show is NBC's "The Office," where 28 percent of its audience watched it sometime other than Thursdays at 9 p.m, Nielsen said. Action shows and serialized dramas, like "Fringe," "Heroes" and "Grey's Anatomy," have big time-shifted audiences. Not surprisingly, young people are the quickest to adapt to new technology.
Among the least time-shifted shows this fall were "Deal or No Deal," "60 Minutes" and "King of the Hill."
With "The Office," time-shifting has kept alive a show that might otherwise be dead. The comedy has the week's toughest time slot, competing directly against CBS' more popular "CSI: Crime Scene Investigation" and ABC's "Grey's Anatomy."
The flip side is that DVRs make it harder for new shows like NBC's just-canceled "My Own Worst Enemy" to get established. Given the choice of trying something new or watching a recorded version of a favorite show, the DVR usually wins out.
"I call the DVR our frien-emy," Wurtzel said.
Time-shifting has played a prominent part in the decline of the 10 p.m. time slot, where a powerhouse like NBC's "ER" ruled television not too long ago. Only three of Nielsen's top 20 prime-time shows a week ago started at that hour, all of them on CBS.
Many viewers are recording shows from 8 or 9 p.m. and watching them later, after dinner or when the kids go to bed, instead of what's on live at 10 p.m. This phenomenon hurts late-night programming, too.
"The biggest single competitor to network programming in any time slot now is (pre-recorded) network programming," said David Poltrack, chief researcher at CBS.
Networks will likely continue to concentrate their top shows in an earlier hour. Some executives can even see a day when networks stop putting high-cost scripted series at 10 p.m. altogether, although there's pressure from local stations to provide strong lead-ins to their late-night news shows.
There was a time, not too long ago, when network executives slept with laptops or fax machines by their beds so they could rise before dawn to check the previous night's ratings.
Now, Ostroff said, "it's a system that's no longer relevant."
She got a peek at the new TV world last spring. CW executives were getting an anecdotal sense that "Gossip Girls" was catching on, even though it wasn't reflected in the overnight ratings. It had a big DVR pickup, and many young fans watched free video streams. The CW briefly stopped streaming the show in order to increase the TV ratings, but fans quickly found illegal versions online, so CW streamed again.
The problem: the CW isn't earning as much from the show as it should, considering how many people are watching it.
"We've got to figure out a way to monetize this content being consumed," Ostroff said.
The networks' weekly ratings scorecard, a traditional psychic barometer, also means less. It's based on live viewing, plus playbacks within 24 hours. One recent week the broadcast networks were down 10 percent from the previous year - an alarming sign of failure on its face - but add in a week's worth of time-shifters and the decline was only 3 percent, Poltrack said.
Asked whether the increased time-shifting helped the networks, Fox chief scheduler Preston Beckman was as ambivalent as Wurtzel.
"It's a little of both," he said. It's always encouraging that viewers watch the shows, whenever they do it. But advertising rates are calculated based on people who watch a show within three days of its original airing. So if you tape "House" on Tuesday to watch Saturday night, Fox gets nothing for it.
He worries that the ease of DVRs may get people out of the habit of watching their favorite shows. First, they don't have to worry about being at the TV at a certain hour because their shows are being recorded. Then they forget to watch the playback. Before you know it, they've stopped seeing the shows regularly.
It isn't simply more houses getting DVRs that is making a difference these days, it's houses getting their second or third DVRs, the experts said.
CBS' Poltrack believes that DVR usage will continue to grow until the machines are in about half of the nation's homes with TVs. He expects the technology to become obsolete soon after that, because more people will have televisions and computers working together to give them even more freedom to program their personal networks.
"We basically have reached the point now where everyone realizes that it's in everyone's best interests to make popular programming available so people can watch it any time they want to watch it," he said.
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