Magician-turned-mathematician uncovers bias in coin flipping
By ESTHER LANDHUIS
Persi Diaconis has spent much of his life turning scams inside out. In 1962, the then 17-year-old sought to stymie a Caribbean casino that was allegedly using shaved dice to boost house odds in games of chance. In the mid-1970s, the upstart statistician exposed some key problems in ESP research and debunked a handful of famed psychics. Now a Stanford professor of mathematics and statistics, Diaconis has turned his attention toward simpler phenomena: determining whether coin flipping is random. Could a simple coin toss -- used routinely to decide which team gets the ball, for instance -- actually be rigged?
Diaconis set out to test what he thought was obvious -- that coin tosses, the currency of fair choices, couldn't be biased. "Mathematicians are always doing that," he says. "You know, everybody knows it's true, and then we prove it. So what, right?"
Wrong. Diaconis had good reason to suspect that surprising truths lurk beneath common assumptions. He had uncovered them time after time. For example, people had long supposed that a few shuffles were sufficient to randomize a deck of cards -- until 1992, when Diaconis and Columbia University's David Bayer proved that thorough mixing requires seven shuffles.