NEW YORK — ABC and NBC promise to improve in prime time’s last hour next season, and last week shows why.

Jerry Seinfeld’s “Marriage Ref” reached fewer than 4 million viewers at 10 p.m. last Thursday, the Nielsen Co. said. That’s a fraction of the 20 million-plus viewers that “ER” used to draw in the same time slot during the 1990s, a key component of NBC’s financial success.

Viewers tuned away from NBC as the night went on. “The Office” had 6.6 million viewers at 9 p.m., “30 Rock” had about 5 million at 9:30 p.m., then “Marriage Ref.”

The effect was more pronounced Wednesday at ABC, where the dead-on-arrival drama “Happy Town” barely reached 3 million. That was less than half the audience for “Cougar Town,” the series that directly preceded it.

No surprise, then, that both ABC and NBC have new shows scheduled for those time slots in the fall.

Singing and dancing competitions again dominated prime time last week. Both editions of “American Idol” and “Dancing With the Stars” had 19 million to 20 million viewers, Nielsen said.

CBS won the week with an average prime-time audience of 10.5 million viewers (6.4 rating, 11 share). ABC averaged 8.6 million (5.5, 10). Fox had 8 million (4.7, 8) but won among the 18-to-49-year-old viewers that it targets. NBC had 5.5 million (3.5, 6), the CW had 2.3 million (1.5, 2) and ION Television had 930,000 (0.6, 1).

NBC’s “Nightly News” topped the evening newscasts with an average of 7.9 million viewers (5.3, 11). ABC’s “World News” was second with 7.3 million (4.9, 10), and the “CBS Evening News” had 5.6 million viewers (3.7, 8).

A ratings point represents 1,149,000 households, or 1 percent of the nation’s estimated 114.9 million TV homes. The share is the percentage of in-use televisions tuned to a given show.

For the week of May 10-16, the top 10 shows, their networks and viewerships: “American Idol” (Wednesday), Fox, 19.57 million; “American Idol” (Tuesday), Fox, 19.17 million; “Dancing With the Stars,” ABC, 19.16 million; “NCIS,” CBS, 17.23 million; “NCIS: Los Angeles,” CBS, 16.04 million; “The Mentalist,” CBS, 14.84 million; “Two and a Half Men,” CBS, 13.86 million; “The Big Bang Theory,” CBS, 13.73 million; “Survivor: Heroes and Villains Finale,” CBS, 13.46 million; “CSI: Crime Scene Investigation,” CBS, 13.42 million.

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