#### 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 = \[\sqrt{{\frac{1}{n}}\displaystyle\sum_{i=1}^{n} (x_i - \bar{x} )} \]