The standard deviation is a statistical measure of the precision in a series of repetitive measurements. It is calculated from:
Where s is the standard deviation, N is the number of datum, xi is each individual measurement, and x bar is the mean of all measurements. The value xi - x bar is called the residual for each measurement.
The standard deviation may also be defined as the positive square root of the variance of the set of measurments. That is the variance is: