The new measurement, dubbed the "Nielsen Twitter TV Rating," seeks to tap into the stream of viewer commentary and armchair musings generated on "second screens" - the smartphones and tablets perched on Twitter users' laps while they watch, say, Monday Night Football or the latest episode of "Homeland" on their TVs.

The new ratings, to be launched next fall, arrive at a moment when media and advertising industry executives say they are observing a shift in TV viewing habits that include the rise of "second screen" use.

But significant questions remain for advertisers over how best to interpret the data and whether a Twitter ratings system is meaningful at all.

In September, Nielsen ratings showed that TV viewership for Viacom Inc's MTV Video Music Awards, which coincided with the Democratic National Convention, plummeted by more than 50 percent from a year ago. Yet social media chatter tripled, according to the research firm Trendrr.

Brad Adgate, an analyst at Horizon Media, said advertisers will view the Twitter ratings as a useful layer of information about a show's popularity, but it is "not going to be close to the currency" of existing ratings metrics.

"It lets producers and creative directors know if the storyline is working, like a huge focus group," Adgate said. "But I don't think you can translate comments to ratings for a show. Right now I think the bark right now is bigger than its bite."

The new ratings will measure the number of people discussing a show on Twitter, as well as those who are exposed to the chatter, to provide the "precise size of the audience and effect of social TV to TV programming," Nielsen said.

"As the experience of TV viewing continues to evolve, our TV partners have consistently asked for one common benchmark from which to measure the engagement of their programming," Chloe Sladden, Twitter's vice president of media, said in a post on the company blog on Monday. "This new metric is intended to answer that request, and to act as a complement and companion to the Nielsen TV rating."

Mark Burnett, executive producer of NBC's hit "The Voice," argued that advertisers should value programs that can attract a high level of social media engagement from viewers. Deeply embedded social media elements, such as live Twitter polls, were critical in driving "The Voice" to the top of the Tuesday night ratings among viewers between 18 to 49, Burnett said.

"If you're an advertiser, wouldn't you want to know whether people are watching this show passively or if they're actively engaged in the viewing experience?" Burnett said. "Five years from now this will make traditional television ratings seem archaic."

For Twitter, the partnership with a recognized measurement company like Nielsen emphatically punctuates a year-long effort by its media division to bring second-screen usage into the mainstream.

Twitter's convergence with television has been on display during sporting and major news events, which have provided some of the biggest viewership moments for both broadcasters and the social media company.

During the Summer Olympics in London, Twitter set up a page for the event that displayed photos from inside an event venue or athletes' tweets to complement what was being broadcast on NBC. Advertisers like Procter & Gamble Co, for instance, which advertised heavily during the Games, tried to bridge the two mediums by airing an ad on TV, then sending out a tweet soliciting viewer feedback about the ad.

As news organizations tallied votes on election night in the United States on November 6, worldwide Twitter chatter hit a peak of more than 327,000 per minute, the company said this month.

(Reporting By Gerry Shih; Editing by Steve Orlofsky and Diane Craft)

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