Los Angeles ? UPN has seen a sharp ratings drop for its six comedies that appeal largely to blacks, and the network isn’t sure whether that reflects a change in their popularity or how viewers are counted.

Five of the six comedies, which air on Monday or Tuesday nights, have seen viewership go down by at least 10 percent since last year, according to Nielsen Media Research.

The single exception: “One on One,” which has benefited from a move to a more desirable Monday time slot. Still, that show’s audience is 11 percent lower than “The Parkers,” which aired in its same time slot last season.

“In all my years doing network television, I’ve never seen a group of six shows come down in a group like this,” said Viacom Co-Chairman and CBS chief Leslie Moonves, who oversees UPN’s programming. (UPN is owned by Viacom.)

None of the comedies are yet threatened by cancellation. But in the television business, lower ratings mean lower advertising revenue, and network executives rarely have much patience for that.

Two of the programs, “Girlfriends” and “Half & Half,” were nominated for NAACP Image Awards for best comedy on Wednesday.

CBS researchers suspect a change in Nielsen’s methodology is to blame. The company has changed its pool of participating families to label more citizens of Caribbean island descent as black. They’re primarily Spanish-speaking and often have different viewing habits than American-born blacks, said David Poltrack, CBS’ top researcher.

“The composition of black viewers in the Nielsen sample has changed,” he said. “I’m not saying it’s right or wrong. It’s different.”

Nielsen has been under fire from some blacks and Latinos in the past year for changing to a new electronic system for counting viewers in big cities. Some critics contend this will undercount minority viewers.

A Nielsen spokesman, Jack Loftus, said there may be something to Poltrack’s theory. But he noted that Nielsen’s numbers have not reflected a change in overall television viewing among blacks.

“We clearly believe in the shows,” said UPN entertainment President Dawn Ostroff. “We feel creatively the shows are in great shape.”

However, she added: “We’re concerned. We want to do everything we can to get the ratings up.”

One thing UPN has succeeded in is changing its audience composition. Three years ago, UPN’s viewership was 47 percent female. Now it’s 56 percent, Moonves said.

One thing that’s helped: the new drama, “Kevin Hill,” starring heartthrob Taye Diggs. UPN’s ratings this fall are up 250 percent among women aged 18 to 34, he said.