standard deviation (SD)
Last reviewed 01/2018
The standard deviation gives the average distance of each of a set of observations from the mean.
It is represented by the Greek letter, sigma.
If the standard deviation is known then it can be stated that in a normally distributed population 68% of values will fall within one standard deviation of the mean, 95% within two and 99% within three standard deviations of the mean.
The standard deviation is the square root of the variance.