Hollywood, where the star system was invented, is not wholly dependent on celebrities: the list of biggest-grossing movies in history is dominated by movies like “Shrek 2,” “ET: The Extra-Terrestrial” and the “Star Wars” series, which were not star-driven. But the industry still places an enormous importance on superstar power based on a straightforward fact: On average, movies that have big names starring in them make more money at the box office than movies that do not.

Movie industry specialists argue that, in the complicated world of Hollywood economics, stars bring many different kinds of benefits. They are easier to market, they help sell more tickets at home and overseas and they help drive home-video sales, which are a bigger and bigger slice of studio revenue. “If the stars’ job is to increase output, by drawing crowds into the theaters or selling DVD’s, it is not working as well as it had worked in the past,” said Harold L. Vogel, author of a book called “Entertainment Industry Economics: A Guide for Financial Analysis.” But, he added: “This is a hiatus. We have gone through 25 years where new distribution for films — videocassettes, cable and DVD’s — added new revenue potential. That meant less resistance to stars’ salary demands.”

Even studio chiefs will acknowledge that a star does not guarantee success. “Bewitched,” starring Nicole Kidman, cost an estimated $85 million and had taken in only about $62 million at the American box office by late 2005. Yet there is a bedrock belief that the winning formula consists of the right star in the right movie.

“If you pay a star a great deal of money for a film that people don’t want to see, then it won’t work,” said Sidney Sheinberg, the former president of MCA Universal. “It is always a question of whether you are dealing with a project that is enhanced by a star or are you dealing with a project where you are looking for the star to make it happen, and sometimes it works and sometimes it doesn’t.”

Anita Elberse, an associate professor at the Harvard Business School, tried to measure the average effect of a star by analyzing casting announcements on the price of stocks on the Hollywood Exchange (www.hsx.com), a simulated market where hundreds of thousands of users trade stocks in individual movies based on their expected box-office revenue. Prices on this exchange have been found to be fairly good predictors of a film’s box-office success.

Ms. Elberse found, for instance, that the announcement in 2002 that Mr. Cruise had dropped out of “Cold Mountain” — he had been expected to play the lead — reduced the movie’s expected gross by $10 million. The announcement that Mr. Cruise was in talks to play a leading role in “The Last Samurai” lifted the movie’s expected gross by $28 million.

Combing through 12,000 casting announcements between November of 2001 and December of 2004, related to 600 movie stars and 500 movies, Ms. Elberse found stars, on average, were worth $3 million in theatrical revenue.