In a year when the combined adults 18-49 rating for the Big Four networks slipped 6 percent compared with last year, the network that slipped the least will once again finish first.

Fox will win its third consecutive season among 18-49s tonight when the regular season ends, and it has done it in dominating fashion. At a 4.0 rating and 11 share, the network’s 0.3 advantage over second-place CBS (3.7/10) is the biggest margin of victory for any network in five years, since NBC in the 2001-2002 season, according to Nielsen data.

Yet even Fox slipped compared with last year, down 2 percent from a 4.1/11. The 4.0 was the lowest average rating to win a season in at least eight years.

Second-place CBS was down 3 percent compared with last year, to a 3.8/10. Third-place ABC slid the most, 13 percent, from a 4.0/11 to a 3.5/9, and NBC is down 6 percent, from a 3.3/9 to a 3.1/8.

Those numbers include sports ratings, such as the World Series, Bowl Championship Series and Super Bowl, which was on ABC last year and CBS this year. Without sports, ABC is down just 5 percent, while CBS is down 8 percent; Fox and NBC’s declines are about the same.

Fox’s victory came once again because of one main reason, “American Idol.” Though the show’s ratings declined compared with last year over its final two months, even a diluted “Idol” is still by far the highest-rated show on television.

The show’s Wednesday edition averaged a 12.3 rating and its Tuesday edition averaged a 12.1, down a combined 3 percent from last year.

But oddly, “Idol” was even more dominant this year than last, as many other shows’ season averages slipped by higher percentages.

Last year, the No. 3 show behind both “Idols,” “Desperate Housewives,” averaged a 9.1, 28 percent behind “Idol.” This year the No. 3 show behind “Idol,” “Grey’s Anatomy,” averaged an 8.1, 34 percent behind “Idol.”

Still, as decisive as Fox’s victory was, it still has a lot of concerns come next year. “24” will end the year down 9 percent from last year, with declines even more steep in recent weeks. “Prison Break,” another action drama, also saw declines, and the network’s Sunday night average fell 14 percent compared with the previous year.

If those problems carry into next season, and “Idol’s” ratings continue to slide, Fox could have a hard time maintaining No. 1.

Meanwhile, in broadcast ratings for the week ended May 20:
 
Among adults 18-49, Fox led with a 3.9 average rating and a 12 share, followed by ABC at 3.6/11, CBS at 2.8/8, NBC at 2.4/7, Univision at 1.7/5, CW at 1.3/4, Telemundo at 0.5/1, MyNetworkTV at 0.4/1, Telefutura at 0.3/1 and Ion and Azteca at 0.1/0.
 
Among adults 18-34, Fox led with a 3.6 average rating and a 12 share, followed by ABC at 2.8/9, CBS, NBC and Univision at 1.9/6, CW at 1.5/5, Telemundo at 0.5/2, MyNetworkTV at 0.4/1, Telefutura at 0.3/1 and Azteca and Ion at 0.1/0.
 
Among adults 25-54, Fox and ABC tied for first, each with a 4.2 average rating and an 11 share, with CBS at 3.5/9, NBC at 2.7/7, Univision at 1.7/5, CW at 1.2/3, MyNetworkTV at 0.5/1, Telemundo at 0.4/1, Telefutura at 0.3/1, Ion at 0.2/0 and Azteca at 0.1/0.
 
Top five (18-49s): 1. Fox’s “American Idol - Wednesday” 10.8; 2. Fox’s “American Idol - Tuesday” 9.4; 3. Fox’s “House” 9.2; 4. ABC’s “Grey’s Anatomy” 9.1; 5. ABC’s “Desperate Housewives” 7.2.
 
Top five (total viewers): 1. Fox’s “American Idol - Wednesday” 28.05 million; 2. Fox’s “American Idol - Tuesday” 24.23 million; 3. ABC’s “Grey’s Anatomy” 22.57 million; 4. Fox’s “House” 21.19 million; 5. CBS’s “CSI” 20.45 million.
 
Bottom five (18-49s): Tie-86. CW’s “Veronica Mars” and NBC’s “Dateline Saturday” 0.9; Tie-88. CW’s “Gimore Girls” and “America’s Next Top Model-Encore” 0.6; 90. CW’s “7th Heaven-Encore” 0.4.
 
Bottom five (total viewers): 86. CW’s “Everybody Hates Chris” 2.34 million; 87. CW’s “Veronica Mars” 2.10 million; 88. CW’s “Gilmore Girls” 1.44 million; 89. CW’s “America’s Next Top Model - Encore” 1.43 million; 90. CW’s “7th Heaven-Encore” 1.18 million.
 
Show on the rise: “The Simpsons,” Fox, Sunday 8 p.m. The show’s 400th episode, which was also its season finale, posted a 4.5 rating among viewers 18-49 and averaged 9.9 million total viewers, the highest in both demos since Jan. 7.
 
Shows on the decline: “Friday Night Smackdown,” CW, Friday 8 p.m. In a week when the CW’s “America’s Next Top Model” and “Gilmore Girls” saw solid jumps, “Smackdown” did not. The wrestle-fest averaged a 1.4 among viewers 18-49 last week, off 13 percent from a 1.6 the previous week.