ATistics> (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: sigma = sqrt(((x1-(avg(x)))^2 + (x1-(avg(x)))^2 + ... + (xn(avg(x)))^2)/n) 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. [dictionary.com ]. ["Barrons Dictionary of MAThemATical Terms, second edition"]. (2003-05-06)