Standard Deviation

For a sample of n numbers with mean x̄, the (sample) standard deviation is s = √( Σ(xᵢ − x̄)² / (n − 1) ). The denominator is n − 1, not n, to give an unbiased estimator of the population standard deviation.

Standard deviation is in the same units as the original data (unlike variance, which is in squared units). Two-thirds of a normal distribution falls within one standard deviation of the mean; 95% within two.