= Probability
{title2=$\mathbb P(E)$}
{wiki}
The probability of an <event> is its value under a <probability measure>, a <real number> between zero and one. It compares the mass assigned to that <event> with the unit mass of the whole <sample space>. For a finite <sample space> with equally likely outcomes, $\mathbb P(A)=|A|/|\Omega|$. For unequal outcome masses, add their individual <probabilities> instead.
For example, selecting a coin with head <probability> $1/2$ with chance $4/5$, or a coin with head <probability> $3/4$ with chance $1/5$, gives head <probability> $(4/5)(1/2)+(1/5)(3/4)=11/20$. This is the <law of total probability>: averaging over the hidden selection is part of calculating the outcome's <probability>.
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