(s,BESD, ) The standard deviation is a statistical measure of precision. The best estimate of the standard deviation s for small data sets is calculated using
where xi is the measurement from the i-th run, x-bar is the mean of all the measurements, and N is the number of measurements. For very large data sets, the standard deviation is the root-mean-square deviation from the true mean, and is usually written as to distinguish it from the best estimate standard deviation s used for small data sets.