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

Yet signs of softening are there. After a huge start to last season, its numbers faltered a bit by spring, and for the first time ever “Idol’s” average among 18-49s slipped year-to-year, by 7 percent.

Fox points out that the decline came after a fifth year in which the show grew several percentage points, meaning it was still roughly even to season four.

But that said, there’s reason to believe “Idol” will be at least as strong as last year, if not even better, following its debut tomorrow night.

The writers’ strike has thrown the networks’ schedules into disarray, forcing a number of reality shows on the air as original episodes of hit dramas and sitcoms run dry. Those shows are not expected to do nearly as well as the scripted shows they replace.

“Idol” always dominates January, but this year its strength could be even more absolute against already low-rated shows such as ABC’s “Just For Laughs” and the CW’s “Crowned: The Mother of All Pageants.”

This year a 30-second spot on the show reportedly will average more than $600,000.

What are advertisers getting for that money?

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

“Idol” finished the past three seasons No. 1 in both adults 18-49 and total viewers. Last season was still one of its best ever, averaging better than 29 million total viewers for the performance and results shows combined, as well as an 11.9 in 18-49s, nearly four points ahead of top scripted dramas, “Grey’s Anatomy” and "House." 

The show’s premiere was the most-watched regularly scheduled broadcast program of the season, averaging a series-record 37.4 million total viewers. The season finale drew 30.7 million, down from the previous year but up quite a bit from the 22.7 million who watched Kelly Clarkson win season one in September 2002.

“The declines are minimal and the series still dominates its competition, remaining the most-watched program on television, partly because it’s one of the few shows families can watch together,” notes giant ad-buying firm Magna Global in an analysis.

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

At $600,000, it’s the priciest regular-season commercial buy by a long shot. ABC’s “Grey’s Anatomy” is No. 2 at a reported $419,000 per spot.

“Idol” was also 2007’s top show in product placement for the second straight year. “Idol” had 4,349 product placement occurrences, according to Nielsen. 

“Idol” reaches a desirable audience. According to Magna Global, the median household income for “Idol” viewers is $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 41 for live viewership of Tuesday’s competition show and 42 for Wednesday’s results show, according to Magna. That’s six and nine 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 609 million votes were cast in the weekly voting that determines who stays and who goes.

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 last year. Last year, season three winner Fantasia made her Broadway debut in “The Color Purple.”

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

Last year “Idol” staged its first-ever “Idol Gives Back” show, in which it raised money for charities both in the U.S. and abroad. More than $60 million in viewer, corporate and Fox-matched donations was raised.