In 2012, U.S. President Barack Obama defeated Republican challenger Mitt Romney to win a second term. Federal finance reports showed campaign expenditures broke the $2 billion mark, making the election the most expensive in U.S. history at the time.

But how do exit polls work, and are they reliable? Here’s what to know on Tuesday, Nov. 5. Exit polls are surveys conducted with voters as they leave their polling place, according to the American Association for Public Opinion Research (AAPOR).