Coin flips are not fair

By | March 2, 2004

Interesting tidbit from Science News: Coin tosses are inherently biased: “‘I don’t care how vigorously you throw it, you can’t toss a coin fairly,’ says Persi Diaconis, a statistician at Stanford University…” Apparently, 51% of the time, coin flips will end up with the same face of the coin that was pointing up when the coin was flipped. In other words, if I put a quarter on my finger with Heads up and flip it, 51% of the time the toss is going to end up Heads.

Asked why this wasn’t discovered before, Diaconis noted that it would take 10,000 tosses for an observer to notice the bias. Since no one in their right mind will sit down and flip a coin 10,000 times, it is easy to miss this bias.

Also interesting, a “spinning penny will land as tails about 80 percent of the time, Diaconis says, because the extra material on the head side shifts the center of mass slightly.”

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