Let $P = (\Omega, \mathcal{F}, \mathbb{P})$ be a D1159: Probability space such that

(i)	$E \in \mathcal{F}$ is an D1716: Event in $P$
(ii)	$I_E : \Omega \to \mathbb{R}$ is an D2796: Indicator random boolean number on $P$ with respect to $E$

Then \begin{equation} \mathbb{E} I_E = \mathbb{P}(E) \end{equation}