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Data & Analytics · Foundational

Standard Deviation (SD)

The square root of variance, describing typical distance from the mean in the data's own units.

Definition

A small standard deviation means values cluster near the mean. In a normal distribution, about 68%, 95% and 99.7% of values fall within 1, 2 and 3 SDs. SQL offers STDDEV_SAMP and STDDEV_POP, and Excel offers STDEV.S and STDEV.P.

Example

Test scores with mean 70 and SD 5: most students scored 65-75.

Exam tip

Use the sample function (STDEV.S, STDDEV_SAMP) when your data are a sample.

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