Published online by Cambridge University Press: 28 February 2022
The gambler's fallacy runs like this: We know
1) the bias for heads of a certain coin and flipping device is 1/2, and
2) flips of the coin with the flipping device are independent.
Given these facts we can easily compute the probability of 4 heads or fewer turning up in 20 flips, P(h≤4), to be
Suppose we flip the coin 15 times and get—improbably enough—all tails. If the coin lands tails up once more in the next 5 flips the improbable event of 4 or fewer heads in 20 flips, h≤4, will have occurred. Since P(h≤4) is so small we might be tempted to infer that all of the next five flips will be heads.