(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)