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Standard Deviation
The standard deviation is a number which is calculated
using a simple mathematical trick (calculating the square root of the
average of squares) to find an 'average' number for the distance of the
majority of measures from the mean.
The standard deviation is usually a preferable method
of measuring spread, as opposed to the simpler 'Range' calculation, as it
takes account of all measurements.
The Greek letter, sigma, (s)
is often used to signify standard deviation.
It is of particular value when used with the Normal
distribution, where known proportions of the measurements fall within one,
two and three standard deviations of the mean, as below.
See also:
Variation, Spread,
Range, Six Sigma
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