Standard deviation
The standard deviation (denoted by the Greek letter σ) gives the degree of variation around the mean value of a stochastic variable. It is calculated as follows:σ2 = ∑ (xi - <x>)2 / (n-1)
where xi (i = 1, 2,… , n) is the result of an individual observation, <x> is the mean value, and n is the number of observations.