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standard deviation (SD)

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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.