Standard deviation

Epic card · No. 9764

In statistics, the standard deviation is a measure of the amount of variation of the values of a variable about its (arithmetic) average. A low standard deviation indicates that the values of a set tend to be close to their average, while a high standard deviation indicates that the values are spread out over a wider range. Standard deviation may be abbreviated SD or std dev, and is most commonly represented in mathematical texts and equations by the lowercase Greek letter σ (sigma). In the case of a normal distribution (as typified by the symmetrical bell-shaped curve), approximately 68.3% of all values fall within one standard deviation of the average, 95.4% within two standard deviations, and 99.7% within three. The standard deviation of a random variable, sample, statistical population, data set or probability distribution is the square root of its variance (the variance being the average of the squared deviations from the mean). A useful property of the standard deviation is that...

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