Big 3 Again Lose Chunk of TV Pie : Ratings: The combined audience of NBC, ABC and CBS declined measurably for the fifth straight season. NBC won its sixth sweepstakes in a row.

The three major networks lost another chunk of their combined audience in the 1990-91 television season, the fifth consecutive year that their viewership has declined measurably, according to figures released Tuesday by the A. C. Nielsen Co.

The aggregate audience for NBC, ABC and CBS dropped nearly 6% from the previous season, meaning an average of 1.7 million fewer households nationwide were tuned to one of the three networks during prime time, despite the fact that the total number of TV households in the United States grew by about 1 million.

The three-network share of the audience during the 30-week season that concluded Sunday slipped to 62.4% of those watching television in prime time, down from 66% in the 1989-90 season and 85% just 10 years ago.

“Cheers,” in its ninth year on NBC, was the season’s top-rated series for the first time. “60 Minutes,” in its 23rd season on CBS, was No. 2.

Rounding out the Top 10 were, in order, “Roseanne,” “A Different World,” “The Cosby Show,” “Monday Night Football,” “America’s Funniest Home Videos,” “Murphy Brown,” “Empty Nest” and a tie between “Designing Women” and “America’s Funniest People.”

The good news for NBC is that it won the network sweepstakes for the sixth straight year in what turned out to be the tightest three-network race in 26 years. The bad news is that NBC’s total audience slid 13% from the year before, which translates to an average loss of 1.7 million households per night. Not one of NBC’s regular programs performed as well as it did last season.

That decline, according to research executives at the other networks, has put NBC’s winning streak in jeopardy.

“NBC came down so dramatically, they are clearly vulnerable,” Arnold Becker, vice president of television research at CBS, said Tuesday. “Now we’re all starting on equal footing, and whoever comes up with the best new shows next season is going to be the winner.”

Not since the 1964-65 season, when only three-tenths of a rating point separated first from third place, has the three-network race been so competitive. NBC finished this season with a 12.7 rating, compared to 12.5 for ABC and 12.3 for CBS. Each point represents 931,000 households. Last year the gap between NBC and third-place CBS was 2.4 points.

Despite trailing, ABC and CBS both had something to gloat about Tuesday.

ABC, which was buoyed by huge ratings for the Super Bowl and the Academy Awards, laid claim to the season victory in terms of total viewers (as opposed to households, to which the Nielsen rating refers). ABC said it delivered an average prime-time audience of 19.2 million viewers, compared to 19 million for NBC and 18 million for CBS. Ranked by total viewers, ABC had five of the Top 10 and 15 of the Top 30 series of the season.

Executives at the two other networks dismissed the significance of these statistics, however, saying that ABC’s programs, especially such shows as “America’s Funniest Home Videos” and “Full House,” are simply watched by more children than their networks’ programs. In prime time, they pointed out, advertisers for the most part buy commercial time in an effort to reach adults.

But ABC also claimed victory among the adult demographic groups most prized by advertisers. For all prime-time programs and for all series programming, ABC attracted the largest number of viewers in the 18-49 and 25-54 age groups.

CBS, which finished last for the fourth consecutive year, was nonetheless cheering the fact that it was the only one of the three networks not to suffer an audience decline. While NBC fell 13% and ABC dropped 3% from last season, CBS improved its mark by one-tenth of a point.

“When the size of the pie shrinks and your piece does not, that is a great improvement,” Becker said. “There is a strong relationship between the share of audience and the share of dollars that you will bill, and so from a competitive point of view, we’re in great shape.”

Fox’s share of the audience, which had grown significantly the past couple of seasons, was flat this past season at 11%. But Fox’s addition of Thursday and Friday night programming last fall clearly hurt the Big 3, driving down their combined share of the audience on those nights to only 60%.

“America’s Funniest People” was far and away the most popular new show of the season. Aside from “Baby Talk” and “Davis Rules,” which to date have had short stints as mid-season replacements on ABC, the next-highest-ranking new show of the season was NBC’s “Fresh Prince of Bel Air” at No. 39. NBC’s “Law and Order” at No. 49 was the highest-ranked new dramatic series.

“Family Matters,” which finished as the 16th-rated series of the season in its sophomore season on ABC, showed the most dramatic improvement of any series on TV, growing 16% over its performance of last year.

Buoyed by the Persian Gulf War and CNN’s nearly 100% growth, basic cable also demonstrated spectacular improvement in prime time, leaping 26% over the previous season. CNN’s prime-time ratings, which skyrocketed during the war, have recently slipped back to where they were in previous seasons. Basic cable also benefited from TNT’s 30% and Lifetime’s 33% increase in prime-time viewership.

PBS, which enjoyed a 14% increase in the size of its audience this season thanks to the success of “The Civil War,” also took a nick out of the Big 3’s pie.

At a news conference in New York, David Poltrack, senior vice president for research and planning at the CBS Broadcast Group, estimated that of the 3.6 points lost in the networks’ share of audience, 1.2 could be attributed to the CNN war coverage, 0.2 to “The Civil War,” 0.2 to new football games on TNT and 2 to “generic erosion.”

While each of the Big 3 networks highlighted the silver linings in the ever-darkening cloud, they left unanswered the big question: Just how low is their share going to go?

At one time in the not-too-distant past, CBS’ Becker said, he had predicted that 63% was as far down as the Big 3’s share would tumble--slightly above where it stands now. While confident that the erosion will bottom out “at some point,” he added, few network research executives are bold enough now to predict the final outcome.