By BILL CARTER and BRIAN STELTER

Published: March 4, 2012

For almost a decade, identifying the most popular show on television has been easy: “American Idol,” case closed.

But that conclusion is not a given anymore, and not only because the numbers for “Idol” have plummeted this winter. The daily ratings are in many ways a mirage now, sure to change significantly once the people who time-shift their television viewing are taken into account.

In fact, because of that behavior, there has been a change in the standings. Among the group that determines much of the revenue of the television business, viewers ages 18 to 49, ABC’s “Modern Family” now is the most popular show on television.

No other show on television comes close to that comedy in adding 18- to 49-year-old viewers who record shows and watch them later. So far this season, new episodes of “Modern Family” have grown from a first-day average of 7.1 million viewers in that age group to 10.2 million, counting seven days’ worth of added viewing — a gain of 3.1 million each week, according to Nielsen Research.

These time-shifters, though, cannot be counted in the overnight ratings, where “Modern Family” tends to trail both “Idol” and NBC’s “The Voice.”

The overnights still “set the tone and the agenda” at the television networks, said Charles Kennedy, the head of research for ABC, influencing marketing and programming decisions. But “we’ve had to build in this fudge factor,” Mr. Kennedy said, “when we know — at least with shows that already have a track record — that the total number will be significantly higher.”

When the television season ends in May, what will matter most — for both networks and advertisers — is the ranking of shows once digital video recorder playback is included in the viewership totals. In a comeback victory for scripted shows, original episodes of “Modern Family” will most likely dethrone “Idol” this season as television’s favorite show for those valuable 18-49-year olds — just as the CBS drama “NCIS” will most likely beat “Idol” for the first time for most overall viewers with its original episodes.

“On behalf of all the comedies that were wiped out by ‘Idol’ over the past 10 years, it’s very gratifying,” said Steve Levitan, one of the creators of “Modern Family.”

Total popularity does not perfectly correlate with profitability, however, since the networks all agree to sell ad time based on a metric called “C3.” It measures the average viewing of the commercials within a show within three days of the first broadcast, so it excludes people who wait to watch Wednesday’s “Modern Family” until Sunday or Monday.

Still, advertisers are paying, happily so, for the three days that are counted.

“We do like viewing in the playback mode,” said Tim Spengler, the global chief executive of the media-buying firm Magna Global. “We’re finding that the viewers are more attentive. They are less distracted. They have picked a time when they have the opportunity for more engagement than they would have if their kids were bugging them or they had three things to do at once.”

Mr. Spengler said many advertisers, like fast food restaurants, movie companies and some retailers, do not want to pay for ads beyond three days because what they have offered might be out of date. But, he said, other advertisers recognize there is “some value” to the four additional days of viewing that are not counted by C3 — even among fast-forwarders, because they do see glimpses of messages here and there.

The networks would eventually like to sell ad time based on seven days of viewership, but most viewership happens in the three-day window; Paul Lee, the president of ABC Entertainment, said ABC is able to “capture about 93 percent” of the value of the “Modern Family” audience with the C3 ratings.

“So we can now truly monetize appointment television,” Mr. Lee said. He noted that in the past, Thursday night shows carried the highest prices in television, because advertisers paid a premium to reach people before their movie openings or weekend car sales.

“Now they buy us on Wednesday,” Mr. Lee said, the day that new “Modern Family” episodes are broadcast, “and they know they are going to get Wednesday, Thursday, Friday and Saturday. We get that C3, and so do they.”

DVRs are now in 43 percent of the American households that have televisions. The growth in DVR ownership is starting to slow down, but other manners of time-shifting — like cable video-on-demand views and Internet stream views — are speeding up, making it harder than ever to assess the total popularity of a show.

Three of the four most-watched TV episodes on Hulu in February were “Modern Family” episodes. All together, Mr. Kennedy said, the video-on-demand and Web streams amount to millions of additional viewers for “Modern Family” each week.

Mr. Lee and Mr. Levitan said that the comeback of the scripted shows this season has something to do with the glut of reality competition shows and the fact that, as Mr. Lee put it, “reality television has lost the shock of the new.”

Those competition shows also tend to be recorded and viewed later much less frequently, so the DVR has been a special enhancement to scripted shows. Among the prime-time hits that get a 40 percent or higher lift among 18- to 49-year-olds because of time-shifting: Fox’s “House,” “Glee,” “New Girl,” and “Alcatraz”; ABC’s “Grey’s Anatomy,” “Private Practice” and “Revenge”; and NBC’s “The Office” and “Up All Night.”

“It used to be that you figured even the most ardent fans of a show saw only two of every four episodes,” Mr. Levitan said. “I don’t think that’s the case anymore. I think with DVR and other ways people can catch up more and more, people actually see the entire season of a show.”

Mr. Spengler can support that hypothesis. He said he and his wife found some time last week and sat down for some viewing. “We watched three straight episodes of ‘Modern Family,’ ” he said.

Mr. Levitan has heard that kind of anecdotal support in even bigger numbers this year, in the wake of his show’s sweep of awards at the Emmys and Golden Globes. He was speaking by telephone from Disneyland, where the show was shooting an episode last week.

“It’s all good,” he said. “I’m literally in the middle of the happiest place on earth and I’m not going to lie to you: I’m one of the happiest guys on earth.”