68-95-99.7 rule
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The 68-95-99.7 rule, or three sigma rule, or empirical rule, states that for a normal distribution, almost all values lie within 3 standard deviations of the mean.
About 68% of the values lie within 1 standard deviation of the mean (or between the mean minus 1 times the standard deviation, and the mean plus 1 times the standard deviation). In statistical notation, this is represented as: μ ± σ.
About 95% of the values lie within 2 standard deviations of the mean (or between the mean minus 2 times the standard deviation, and the mean plus 2 times the standard deviation). The statistical notation for this is: μ ± 2σ.
Almost all (actually, 99.7%) of the values lie within 3 standard deviations of the mean (or between the mean minus 3 times the standard deviation and the mean plus 3 times the standard deviation). Statisticians use the following notation to represent this: μ ± 3σ.
This rule is often used to quickly get a rough estimate of something's probability, given its standard deviation.