The calculation of an estimate of the standard deviation of the population that corrects for bias when using the data in a sample. If you use the sample data to calculate the simple formula for the standard deviation (s.d., σ), this slightly underestimates the true standard deviation of the population. This is because the mean (μ) also has to be calculated from the sample. It is possible to model the bias and then the bias can be corrected by dividing the sum of squares by N–1 instead of N.

standard deviation of the sample = √ ( &;Sigma (x_{i}–μ)^{2}))/N–1

Used on pages 30, 39