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  Standard Deviation advert image shown if present

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A measure of the variation, or variability, in data relative to the mean value of the data. It identifies how far some values might be from the mean of the data set. This method helps to locate values within a normal distribution (also called the empirical rule with 3 standard deviations of the mean), with this being as follows:
• 68.27% of values would be within 1 standard deviation of the mean.
• 95.45% of values would be within 2 standard deviations of the mean.
• 99.73% of values would be within 3 standard deviation of the mean.


M. W. Toews, CC BY 2.5 via Wikimedia Commons, https://upload.wikimedia.org/wikipedia/commons/8/8c/Standard_deviation_diagram.svg

The next, fourth, distribution is 99.994% of values would be within 4 standard deviation of the mean, and there are others as well.