Does a flipped coin have equal odds for heads and tails? Apparently not. Statistician Persi Diaconis, featured in this story on All Things Considered, found that coin flipping isn’t as random as we assume. Using high speed cameras and equations, Diaconis found that even though humans are largely unpredictable coin flippers, there’s a built-in a bias: If a coin starts out heads, it ends up heads when caught more often than it does tails. There’s an approximate 1% bias in favor of heads in this case.
One interesting observation in the report is that coins don’t always flip end over end when we flip them. Sometimes they don’t really flip over at all. Sometimes they tumble in complicated ways. Diaconis’ team constructed a mechanical coin flipper which, when carefully configured, would flip coins with exactly the same outcome each time. We can’t really control our hands to be as precise. So humans are the random element in a coin flip but we’re not as random as we think.
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