<P> In statistics, the standard deviation (SD, also represented by the Greek letter sigma σ or the Latin letter s) is a measure that is used to quantify the amount of variation or dispersion of a set of data values . A low standard deviation indicates that the data points tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range of values . </P> <P> The standard deviation of a random variable, statistical population, data set, or probability distribution is the square root of its variance . It is algebraically simpler, though in practice less robust, than the average absolute deviation . A useful property of the standard deviation is that, unlike the variance, it is expressed in the same units as the data . There are also other measures of deviation from the norm, including average absolute deviation, which provide different mathematical properties from standard deviation . </P>

As a statistical unit of measure what does sigma represents
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