Forget focus groups: Looking at the brainwaves of people watching movie trailers can give "surprisingly accurate" information about how well it will do at the box office, according to a new study from Northwestern.

At the center of the study is a new technique developed by two Northwestern neuroscientists that monitors people's brainwaves to measure their "level of engagement" with an advertisement.

"It turns out, when our brains are truly engaged with the content we are watching, they essentially look the same as one another," Sam Barnett, a Ph.D. researcher, told Northwestern Now. So if you measure the similarity between test subjects, you can tell when their brains are engaged, and the study of 122 moviegoers found this correlates with higher ticket sales.

The researchers claim that their method can predict ticket sales with 20% higher accuracy than focus groups. One potential reason is because it can eliminate recall bias.

"People are probably going to remember a trailer for movies like 'X-Men' or 'Spiderman' best because they are already familiar with the characters," Barnett said. "But with our method, we are not only testing their memory, but also how engaged they feel with the content of the advertisement as it's playing."

According to the researchers, this method could be used not only to help predict ticket sales, but also to workshop trailer cuts to maximize the chance of the movie doing well.

Read more about the study over at Northwestern Now.