The expectation E(X) of a discrete random variable X is an indication of the ‘centre’ of the distribution of X. Another important quantity associated with X is the ‘variance’ of X, and this is a measure of the degree of dispersion of X about its expectation E(X).

Definition 2.32 The variance var(X) of a discrete random variable X is defined by var(X) = E

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[X − E(X)]2

. (2.33)