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Number-crunching in the film industry is legendary for its malleability. So when trying to determine which film cost the most or which franchise is the most successful, finding consistent criteria is essential. But as creative as the accounting may sometimes be, at the end of the day, movie studios are just like any other business: looking for maximum return on their investment.

Which is why if "Return on Investment," or ROI, is used as the measuring stick, the most profitable film in history is Paranormal Activity. ROI is equal to the gross box office income, minus the budget of the movie, divided by the budget, then multiplied by 100.

This may surprise those who think box-office behemoths such as Avatar (the all-time box office champ ) or the Lord of the Rings films would be the most profitable. But those blockbusters often carry enormous production price tags.

Avatar, for instance, cost somewhere between $280 and $500 million to make, depending on who you ask. So while heavily-promoted movies such as Iron Man 2 earn big money, in terms of ROI, they pale in comparison to that cherished Hollywood entity, the low-budget sleeper hit.

Paranormal Activity, Paramount Picture's smash hit scary movie from 2009 earned an astounding $193.2 million worldwide, according to Box Office Mojo, from a production budget of just $15,000 (Note to independent filmmakers: It always helps keep the budget down when you use handheld camcorders and unknown actors).

The ROI of Paranormal Activity ($193.2 million - $15,000/$15,000 x 100) equals a mind-boggling 1,288,360 percent, making it the most profitable movie ever made by a long shot.

The movie even beat out another a low-budget, independent horror movie , The Blair Witch Project. That 1999 film earned $248 million globally. With a $60,000 budget, it earned an enormous ROI of 433,000 percent.

Both The Blair Witch Project and Paranormal Activity are prime examples of how profitable capturing cinematic lightning in a bottle can be.

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