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Used for questions related to statistical measure "variance", i.e. a second central moment of a random variable. The variance is a risk measure.

Variance of a random variable $X$ is defined as $$ \text{VAR}(X) = \text{E}(X-\text{E}X)^2 = \text{E}X^2 - (\text{E}X)^2 $$

The measure tells how a realization of random variable $X$ is dispersed.

Square root of variance is called standard deviation and it is often used as a risk measure. The higher the standard deviation, the higher the risk.