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Random Variables and Discrete Probability Distributions

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Variance <strong>and</strong> St<strong>and</strong>ard Deviation for<br />

a <strong>Discrete</strong> <strong>R<strong>and</strong>om</strong> Variable<br />

• The variance of a discrete r<strong>and</strong>om<br />

variable X, denoted σ2 X = Var(X), Var(X),<br />

is<br />

determined using the formula<br />

σ2 X = Var(X)= Var(X)=<br />

∑[( [(x x – μX) 2 · P(X = x)]<br />

= ∑[x2 · P(X = x)] – (μX ) 2<br />

• The st<strong>and</strong>ard deviation of a discrete<br />

r<strong>and</strong>om variable X, denoted σX, , is the<br />

square root of the variance.

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