# standard deviation

*(in statistics)*a measure of the scatter of observations about their arithmetic mean, which is calculated from the square root of the variance of the readings in the series. The arithmetic sum of the amounts by which each observation varies from the mean must be zero, but if these variations are squared before being summated, a positive value is obtained: the mean of this value is the variance. In practice a more reliable estimate of variance is obtained by dividing the sum of the squared deviations by one*less*than the total number of observations.*See also*significance.* * *

(SD) in statistics a measure of the amount by which each value deviates from the mean; equal to the square root of the variance, i.e., the square root of the average of the squared deviations from the mean. It is the most commonly used measure of dispersion of statistical data. Called also*population standard d.*Symbol σ. See figure at*normal distribution,*and see also*sample standard d.*

*Medical dictionary.
2011.*

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