If X is a finite random variable taking on values x1, x2, . . ., xn, themeanorexpected valueof X, written μ, or E(X), is

μ = E(X) = x1.P(X = x1) + x2.P(X = x2) + . . . + xn.P(X = xn) = ∑ (xi.P(X = xi))If X is a random variable, itsvarianceis defined to be

σ2 = E( [X - μ]2 ). Itsstandard deviationis defined to be the square root σ of the variance.

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