If you think we've had too many movies this year with the figures 3 or 4 in the title, brace yourself: plenty more are on the way.

That condition peculiar to the Hollywood studios- sequelitis- shows no sign of abating. Yet more instalments of the studios' biggest franchises are in the pipeline, which may prompt the question: How many sequels are too many?

Well, if the studios won't make that call, audiences will. As Tom Ortenberg, president of theatrical films at Lionsgate puts it, "The marketplace decides." Lionsgate is a believer in the "more is best" philosophy, as it will release Saw IV in the US in October and Sylvester Stallone's return to his one-man-army character, John Rambo, next year.

The upside for audiences — and studios - with sequels is that they can put familiar characters into different situations, and introduce new characters and fresh plot twists, like the various iterations of Pirates of the Caribbean and the Jason Bourne adventures.

No one would deny that franchises based on books (eg Harry Potter, The Lord of the Rings, James Bond) have a worthy place in cinema. And sci-fi fans clearly supported Star Wars and Star Trek through their various cycles.

The downside is that what starts out as a good idea or concept is turned into a lousy idea or concept, an obvious attempt to milk something that didn't lend itself to being exploited over and over again. Guilty as charged are such flops as Son of the Mask, Caddyshack II, Deuce Bigalow: European Gigolo, Dumb and Dumberer and Book of Shadows - Blair Witch 2.

And did the world really need Scooby Doo 2, Agent Cody Banks 2, Grease 2, Dirty Dancing Havana Nights, Miss Congeniality 2 and George of the Jungle 2?

There are so many variables that go into what makes an entertaining, value-for-money movie that it's often difficult- and sometimes impossible- to capture all of them again, add fresh twists and turns, and achieve the same level of success, commercially and artistically.

Exhibit A in the US this summer was Evan Almighty, the follow-up to Jim Carrey's 2003 hit Bruce Almighty. It stars Steve Carell as Evan Baxter, a television anchorman who gets elected to the US Congress. He vows to "change the world," but he's a hypocrite: He drives a gas-guzzling Hummer, buys a McMansion and agrees to sponsor a massive land-grab bill he hasn't yet read.

As the film opens, Baxter's wife tells him she has prayed that their family will grow closer. Before bedtime, Baxter gets on his knees, and, after expressing thanks for his new home and car, he calls on God to "please help me change the world."

The next day God (played by Morgan Freeman) appears at Baxter's doorstep, as do truckloads of timber and a copy of "Ark Building for Dummies." Pairs of animals start following Baxter, and the clean-shaven congressman quickly starts looking like Charlton Heston in The Ten Commandments.

The suspicion that Evan may need a miracle to have any chance of recouping its US$175 million production cost was aroused when 20th Century Fox, the US distributor, attempted to market the film to the country's Bible Belt.

The studio partnered with Grace Hill Media, publicity and marketing firm that helps to connect Hollywood studios with the nation's estimated 200,000 churches and millions of worshippers. The campaign was in vain, judging by the mediocre box-office opening and generally dire reviews. The film opens in Australia on September 20.

Nonetheless, Hollywood's scorecard with sequels this year is very much on the right side of the ledger, after hits such as Shrek the Third, Spider-Man 3, the latest Harry Potter and Pirates of the Caribbean adventures, Rush Hour 3 and Die Hard 4.0.

Looking back, it's hard to imagine a time when sequels were not in vogue. "'Sequel' was a dirty word," says Paul Dergarabedian, president of the box-office tracker Media by Numbers. "They were seen as an exploitive way to capitalize on brand name recognition of a popular movie by making the same movie and putting a `2' behind it."

That mindset began to change in 1999 when Austin Powers: The Spy Who Shagged Me made more money in its first weekend in the US than the original managed to do in its entire theatrical run.

Business Week has identified three components that it says almost guarantee the success of the biggest franchises, along with the pre-requisite bankable stars and massive budgets and advertising campaigns: A story line that allows the studio to extend the franchise beyond the initial hit (so no killing off the hero in the first episode); smart merchandising plans, so that every supermarket is packed with Shrek, Spidey, Harry, or Captain Jack Sparrow products; and they know their audience and target it like a laser.

Peter Bart, editor-in-chief of Hollywood trade paper Variety, credits the studios with improving the quality of their franchise sequels. "The studios have learned how to win at this business," he says. "They've picked the right subjects and understood how to exploit them internationally."

But as the sequel numbers pile up, Bart says, the studios are moving into "new waters." Every time they go back to the well, the riskier it gets to depend on the same franchises. "The big question," he says, "is how long will they endure?"

A fourth Shrek film is scheduled for release in 2010 and Jeffrey Katzenberg, chief executive of DreamWorks Animation, has said a fifth will follow

One of the most anticipated sequels is the fourth Indiana Jones, starring Shia La Beouf as the young Indie, Harrison Ford returning and Steven Spielberg directing. Sean Connery declined to participate.

There have been rumblings of a fourth Ocean's movie following Ocean's Thirteen. The Bourne Ultimatum was based on Robert Ludlum's third and final Bourne novel, but author Eric Van Lustbader was commissioned to write two more books, The Bourne Legacy and The Bourne Betrayal.

Transformers, which was based on the toys and 1980s animated TV show, has racked up impressive grosses around the world and seems destined for at least one sequel.

Director Ron Howard and his producer partner Brian Grazer are planning a pre-quel to their critically-maligned but hugely successful The Da Vinci Code.

You can bet Spider-Man will fly again. What's not so certain is whether Sam Raimi will direct again. "It's a very logical thing to think that we can look forward to another one," he has said, without committing himself.

Writer-director Frank Miller has hinted there will be a sequel to his historical epic, 300, which used CGI in an innovative way to keep the budget at a relatively modest $US70 million, and he is preparing Sin City 2 and 3.

To no one's surprise, Disney has announced it will make every one of the seven Narnia novels into films, and they'll be released once a year.