Standard Deviation. The standard deviation (this term was first used by Pearson, 1894) is a commonly-used measure of variation. The standard deviation of a population of values is computed as:

= [(xi-µ)2/N]1/2

µ     is the population mean
N    is the population size.
The sample estimate of the population standard deviation is computed as:

s = [(xi-x-bar)2/n-1]1/2

xbar   is the sample mean
n        is the sample size.

See also, Descriptive Statistics Overview.