# Variance and Standard Deviation - Standard Deviation

#### notes

The units of individual observations x_i and the unit of their mean x are different from that of variance, since variance involves the sum of squares of  (x_i–  bar x ).  The  proper measure of dispersion about the mean of a set of observations is expressed as positive square-root of the variance and is called standard deviation. Therefore, the standard deviation, usually denoted by σ , is given by
σ =   $\sqrt{{\frac{1}{n}}\displaystyle\sum_{i=1}^{n} (x_i - \bar{x} )}$

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