Standard deviation

Standard deviation (symbol σ) is a measure of the dispersion of a set of numerical values. Intuitively, it can be thought of as the mean deviance from the average. It is calculated by taking the square root of variance, or

$\sigma ={\sqrt {{\frac {1}{N}}\sum _{i=1}^{N}(x_{i}-\mu )^{2}}},\mu ={\frac {1}{N}}\sum _{i=1}^{N}x_{i}.$

where x_i is the value of the ith data point in a set and N is the total number of data points.