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variance

How spread out a list of numbers is around its own mean, $\sigma^2=\frac1n\sum(x_i-\mu)^2$ ; its square root, $\sigma$ , is the standard deviation.

Examples

For $z=(1,2,3,4)$ : mean $2.5$ , variance $\frac{2.25+0.25+0.25+2.25}{4}=1.25$ .