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Suppose the discrete variable X takes the values x where the x belong to some set A. So, for example, if X is the result of a roll of a die then A = {1, 2, 3, 4, 5, 6}.

Then the expected value of X,

E(X) = Sum of [x*Prob(X = x)], where the summation is over all x in A.

In the die example, this would be

1*Prob(X=1) + 2*Prob(X=2) + ... + 6*Prob(X=6)

Let E(X2) = Sum of [x2*Prob(X = x)], where again the summation is over all x in A.

Then Variance(X) = E(X2) - [E(X)]2

and the Standard Deviation = sqrt(Variance).

If X is a continuous variable, then replace Prob(X = x) by the probability density function of X and instead of the summation over A, you will need to integrate with respect to x over the set A.

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Q: How do you calculate the expectation value and standard deviation when you have multiple probabilities?
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