<statistics> (SD) A measure of the range of values in a set of
numbers. Standard deviation is a statistic used as a measure
of the dispersion or variation in a distribution, equal to the
square root of the arithmetic mean of the squares of the
deviations from the arithmetic mean.

The standard deviation of a random variable or list of numbers
(the lowercase greek sigma) is the square of the variance.
The standard deviation of the list x1, x2, x3...xn is given by
the formula:

The formula is used when all of the values in the population
are known. If the values x1...xn are a random sample chosen
from the population, then the sample Standard Deviation is
calculated with same formula, except that (n-1) is used as the
denominator.