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entropy

The average self-information of a random variable — how uncertain its outcome is, in bits, before it's revealed.

For $X$ over a finite alphabet with pmf $p$ , $H(X) = -\sum_i p(x_i)\log_2 p(x_i)$ .

Examples

A fair coin has $H=1$ bit; a coin that always lands heads has $H=0$ .