Definition:Variance/Discrete/Definition 2
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Definition
Let $X$ be a discrete random variable.
Then the variance of $X$, written $\var X$, is defined as:
- $\ds \var X := \sum_{x \mathop \in \Omega_X} \paren {x - \mu^2} \map \Pr {X = x}$
where:
- $\mu := \expect X$ is the expectation of $X$
- $\Omega_X$ is the image of $X$
- $\map \Pr {X = x}$ is the probability mass function of $X$.
Also see
Sources
- 1986: Geoffrey Grimmett and Dominic Welsh: Probability: An Introduction ... (previous) ... (next): $\S 2.4$: Expectation: $(22)$