Number of Viewers Watching Prime-Time Television Drops : Television: The drop in audience can cost the networks millions in revenue.

The number of households watching prime-time television during the 1989-90 season was 2% lower than the year before, analysts at the networks said Tuesday. And the falloff in daytime is even more dramatic: Between February, 1989 and February, 1990, viewing dropped 9% overall and 13% among women in the key age group of 18 to 49.

“A decline of 2% by itself is not that significant, but it’s (the decline) affecting every day-part and has been getting worse since January,” Larry Hyams, the director of research for ABC, said in an interview. Noting the daytime drop, Hyams said, “When you start losing 13% of the audience, that can cost the networks millions of dollars in lost revenue. . . .”

Network research executives were at a loss Tuesday to explain the drop and said that they were not convinced that the numbers reflected an actual loss of viewers. The networks have asked the A.C. Nielsen ratings company to provide more data about how it gathered the information.

“This is an industry-wide problem that affects everybody,” Hyams said, “and nobody has been able to come up with a good explanation.” He speculated that new additions to the Nielsen sample or changes in the “people-meter” methodology of measuring audiences might be at least partially responsible for the drop in TV use, which has been most pronounced since January.

In a prepared statement, Nielsen officials said that they had “undertaken an extensive analysis” of audience-measuring procedures and that the findings to date “provide no indication of any fault in the Nielsen system.”

During the 1989-1990 season, the number of households using television (called “HUT” levels) averaged a 60.3 rating during prime time, according to Nielsen data, which means that 55,540,000 homes were watching prime-time at any one time. During the 1988-1989 season, the HUT rating was a 61.5, or 55,600,000 homes.

The news about the dropoff in HUT levels came to light as ABC, CBS and NBC were announcing the results of the so-called “official,” 30-week prime-time season, which ended Sunday. NBC won for the fifth consecutive season with a 14.6 rating and a 24 share, ABC came in second with a 12.9 rating and a 21 share and CBS was third place, with a 12.2 rating and a 20 share.

There was a discrepancy among the networks about what had been the most popular series of the season. CBS gave the nod to NBC’s “The Cosby Show” while NBC and ABC said it was “Roseanne.” The difference was that CBS counted a “Roseanne” episode that was broadcast outside of its regular time period. Rounding out the Top 10, in order, were “Cheers,” “A Different World,” “America’s Funniest Home Videos,” “The Golden Girls,” “60 Minutes,” “The Wonder Years,” “Empty Nest” and “Monday Night Football.”

The three-network share in prime-time declined from 67% last year to 66%. NBC’s ratings were down 9%, CBS’ by 4%, while ABC held steady from a year ago.

Although the Fox network has made inroads against the other three networks, particularly with its Sunday-night lineup of “The Simpsons” and “Married . . . With Children,” David Poltrack, the vice president in charge of research at CBS, said at a news conference, “The decline in HUT levels is the greatest culprit” in the networks’ decline in prime-time share. Although Poltrak said that he suspected statistical methodology as a cause for the drop in overall viewing in recent months, he said that the programming may also be a factor. “The remedy to the decline in HUT levels is to get some good new shows,” he said.