Forget who's No. 1. We compute the summer hits with staying power.

Our long national summer movie nightmare is over. Not that anyone in Hollywood feels your pain: Despite wretched reviews for most of this season’s blockbusters, the studios still managed to get as plump as Julia Roberts in her America’s Sweethearts fat suit. Overall box office hit $2.96 billion, up 10.4 percent from last summer (and that doesn’t include the currency from worldwide grosses, video rentals, and DVD sales). Plus, there was no shortage of records broken: American Pie 2 had the best opening for an R-rated comedy ($45.1 million). Rush Hour 2 posted August’s biggest bow ($67.4 million). Planet of the Apes had the highest non-holiday opening ($68.5 million). And Legally Blonde ($20.4 million) scored the best debut for a film that’s— um—like super, super pink.

But this season was truly a roller-coaster ride, with box office highs followed by ego-shattering lows. (Apes‘ gross dropped 60 percent in its second weekend.) All told, 8 of the 12 films to open at No. 1 plummeted 50 percent or more in their sophomore outings. What happened? Part of the blame rests with studio marketing that focuses exclusively on the opening weekend (after which TV ads virtually disappear). Another factor: the overabundance of megaplexes, allowing movies like Apes to unspool on nearly 5,000 screens. ”When you open at such huge numbers, it’s impossible not to have substantial drops,” says The Fast and the Furious producer Neal Moritz.

The good news is we’ve seen the last of what the suits call ”front-loading,” at least for a while. Most experts say it won’t spread beyond the hypercompetitive summer, when studios earn about 40 percent of their yearly revenue. ”This is high season for event movies,” says Tom Borys, prez of box office tracking firm ACNielsen EDI. ”It’s when the most audience is available.”

But with such a quick-hit summer, the old formulas for judging receipts no longer apply. Don’t get us wrong: A film’s ability to debut at No. 1 with a big gross remains paramount to Hollywood’s bottom line. But this summer it wasn’t a real indication of a movie’s actual appeal to moviegoers. Now that a studio can plaster the world with ads and orchestrate a record-breaking debut, being No. 1 seems more meaningless. It’s almost as if a new formula is needed to judge a true blockbuster in an era of one-weekend wonders.

That’s where we come in. ENTERTAINMENT WEEKLY has devised the Film Legs Index (FLI) to determine which summer 2001 films got people buzzing and buying tickets after the marketing come-on for that crucial opening weekend. By using our formula—for you cine-mathematicians out there, divide the film’s total gross by its opening haul—we’ve uncovered how many times a movie was able to replicate that inflated first-weekend gross in its lifetime in theaters. And the results pack more surprises than The Others.

Consider that ogre-achiever Shrek, which opened at $42.3 million and has grossed more than six times that figure since then. A sure sign of a massive hit with legs—it’s as if each person who went that first weekend managed to get five friends to see it later on. ”You [got] the feeling ‘Hey, people really like this movie!”’ says Sony marketing head Jeff Blake, pointing to Shrek and The Fast and the Furious. (Of course, this formula tends to favor kid-friendly films—like Atlantis: The Lost Empire and Dr. Dolittle 2—that often draw repeat business and strong attendance on weekdays.)