The standard deviation is the most generally useful measure of the variability, or spread, of a set of values. It indicates the extent of the scattering of the values around the mean.
Computationally, the standard deviation is the square root of the variance. However, the standard deviation is easier to interpret than the variance because it is expressed in the same units of measurement as the data, rather than in squared units.
At least two nonmissing values are required for computing the standard deviation.