Standard Deviation (from Internet Glossary of Statistical Terms)

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Standard Deviation
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The standard deviation is one of several indices of variability that statisticians use to characterize the dispersion among the measures in a given population.

To calculate the standard deviation of a population it is first necessary to calculate that population's variance. Numerically, the standard deviation is the square root of the variance. Unlike the variance, which is a somewhat abstract measure of variability, the standard deviation can be readily conceptualized as a distance along the scale of measurement.

Calculating the Std. Deviation