Standard deviation is a statistic that measures the variability or dispersion of a set of data. It is calculated from the deviations (distances) between each data value and the sample mean, and is often represented by the letter "s." The more disperse the data is, the larger the standard deviation. The standard deviation squared is called the variance.
The standard deviation of a population of values is computed as follows:
is the population mean
N is the population size
The sample estimate of the population standard deviation is computed as follows:
xbar is the sample mean
n is the sample size