standard deviation

- noun a measure of the amount by which a set of values differs from the arithmetic mean

- A statistical measure of the dispersion or spread around the
__mean__, for a given set, series, or population. It is__equal__to the__square____root__of the__variance__. If a__data set__has as a bell-shaped distribution, for instance, 68.3% of the data will fall on either side of one standard deviation,__95__.5% within two standard deviation, 99.7% within three, and so on. Its abbreviation is SD. Its symbol is σ.

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