The question each January, as Fox readies the premiere of its mega-hit reality show “American Idol,” is whether “Idol’s” hold on viewers will finally begin to soften.

It’s been the No. 1 show among adults 18-49 a record five straight seasons, the most since demographic ratings were introduced 20 years ago, pushing Fox to first place in the demo the past four years.

Yet signs of softening are there.

For the second straight year, “Idol” declined among adults 18-49, with its Tuesday edition slipping 9 percent, from an 11.7 rating to a 10.7, and its Wednesday edition falling 14 percent, from a 12.0 to a 10.3.

That’s despite airing opposite mostly writers’ strike filler during the first half of its season, before scripted programming returned to the air. This spring, “Idol” faces much tougher competition, including, in tomorrow’s two-hour opener, CBS’s surging first-year hit “The Mentalist,” which last week drew nearly 20 million total viewers.

There have also been a number of behind-the-scenes changes, some of them designed to combat last year’s viewer decline. Gone are the low-rated “Idol Gives Back” charity special and executive producer Nigel Lythgoe, who left voluntarily after a seven-year run with the show.

“Idol” will put less emphasis on the opening audition sequences and more on the Hollywood competition, with the semifinalist field expanding from 24 to 36.

Perhaps the riskiest change is the addition of a fourth judge, songwriter Kara DioGuardi, alongside Paula Abdul, Simon Cowell and Randy Jackson. Cast additions in the eighth year of any show always have the smack of desperation about them – witness “The Brady Bunch’s” show-killing cousin Oliver.

“Idol” will risk alienating its core fan base if DioGuardi does not meld seamlessly into the already-established group.

Still, all that said, “Idol” is expected to once again dominate primetime. Ratings have decline for nearly every hit show this season, from ABC’s “Dancing with the Stars” to NBC’s “Heroes” to Fox’s own “House.” “Idol” was so far ahead of them to begin with that it could fall by double-digits and still easily be TV’s No. 1 show.

This year a 30-second spot on the show reportedly will average more than $700,000.

What are advertisers getting for that money?

On the day before “Idol’s” eighth-season premiere, here’s a quick look at other things media buyers and planners need to know about “Idol.”

“Idol” finished the past four seasons No. 1 in both adults 18-49 and total viewers. Last season it averaged 27.3 million total viewers for the performance and results shows combined, or 6 million better than the second-place “Stars.” It was more than four points ahead of top scripted show “Desperate Housewives” among 18-49s.

The show’s premiere was the most-watched regularly scheduled broadcast program of the season, averaging 33.4 million viewers, though that was down from the previous year’s record bow of more than 37 million.

The finale drew 31.7 million total viewers, up 1 million from the previous year and up quite a bit from the 22.7 million who watched Kelly Clarkson win season one in September 2002.

Fox charged a reported $1.3 million per 30-second ad for May’s “Idol” finale, the same as the previous two years and more than CBS got for the “Everybody Loves Raymond” series-ender in 2005.

 “Idol” was also 2007-‘08’s top show in product placement for the second straight year. It had 4,636 product placement occurrences, according to Nielsen. 

“Idol” reaches a desirable audience. According to Magna Global, the median household income for “Idol” viewers in 2007 was $63,000, up slightly from the previous year and just $5,000 behind top shows like “Boston Legal” and “60 Minutes.”

What’s more, about two thirds of “Idol” viewers live in households with three or more people, meaning the show draws a large family audience.

It also does well among minorities. “Idol” ranked tops with Hispanic and black viewers last season in 18-49s.

The median age of “Idol” viewers has crept up to 42.7 for live viewership of Tuesday’s competition show and 43.4 for Wednesday’s results show, according to Horizon Media, an all-time high. That’s eight and 10 years older than when the show started, but it’s a desirable graying. It’s helped attract more upscale advertisers to the show.

Viewers are downright rabid in their devotion to the show. Last season, more than 97.5 million votes were cast in the finale voting, with David Cook emerging as the winner.

Several “Idol” singers have released platinum albums, including winners Clarkson and Carrie Underwood and season two runner-up Clay Aiken. Clarkson and Underwood have both won Grammys, while season three finalist Jennifer Hudson earned an Academy Award for best supporting actress two years ago.

Others haven’t been as lucky. Season five winner and runner-up Taylor Hicks and Katharine McPhee were dropped by their labels, as was season two winner Ruben Studdard.