Coin Flipping Isn’t Fair

According to Stanford statisticians, coins may not be as fifty-fifty as we think.

Tossing a coin has long been the cheapest, fastest, and fairest way to settle a score. Everything from playground disputes to Super Bowl kickoffs are decided by calling heads or tails. But coins may not be as fifty-fifty as we think. Three Stanford statisticians concluded that a flipped coin caught in midair has a 51 percent chance of landing the same way it started. Their data comes from a mere 27 flips, recorded with a high-speed camera in a Stanford lab. It would take 250,000 flips to experimentally confirm such a small bias, but for now, the authors advise giving coins a vigorous pre-flip palm shaking to randomize the coin's starting position.

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