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Standard Deviation

A common measure of the spread of data.

xˉ\bar{x} is the average of some numbers x1,...,xnx_1, ..., x_n

Standard deviation of those numbers is

s=1n∑i=1n(xi−xˉ)2s = \sqrt{\frac{1}{n} \sum\limits^{n}_{i=1} (x_i - \bar{x})^2 }