A measure of the variability of a data set. For a simple data set with numbers, can be calculated using the following formula:

$$\sigma ={({(\sum ({\text{x-x}}_{\text{m}}))}^{2}/\text{N})}^{0.5}$$

- σ is standard deviation.
- x
_{m}is the average. - Σ(x-x
_{m}) is the sum of x_{m}subtracted from each individual number x. - N is the total number of values.

Note: Other formulas also exist.