In more detail
About 68.27%, 95.45% and 99.73% of values fall within 1, 2 and 3 standard deviations of the mean. Averages of many independent values tend towards a normal shape, whatever the original distribution.
Glossary
Math & Statistics
Normal distribution
The normal distribution is the symmetric bell-shaped curve that describes many natural measurements, defined by its mean and standard deviation.
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See also
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Glossary
z-score
A z-score says how many standard deviations a value lies above or below the mean: `z = (x − mean) / SD`.
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Glossary
Standard deviation
Standard deviation measures how far values typically stray from their mean; it is the square root of the variance.