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Snap Judgment: When to Trust Your Instincts, When to Ignore Them, and How to Avoid Making Big Mistakes with Your Money by David E. Adler

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21. The Truth About Coin Tosses: They Aren’t Fair

Heads or tails. How to call it? Do you think it’s random, unpredictable? We use coin tosses to settle disputes and decide outcomes, because we believe they are unbiased with 50/50 odds. And we use them metaphorically, too. “It’s a coin toss” means something could go either way.

Our bias is that coin tosses are fair. But they aren’t—they have a bias. The odds are not 50/50 heads/tails. Instead, a tossed coin tends to come up the same side as it started. Heads facing up predicts heads; tails facing up predicts tails. So much for our intuition. The statistical differences involved are too subtle for us to notice, but they are real nonetheless. Three academics—Persi Diaconis, Susan Holmes, and Richard ...

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