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Standard deviation is a commonly used measure of spread. It is a measure of how much a randomly selected value from a data set is expected to differ from the mean. To denote the standard deviation, the Greek letter $\sigma$ is used, which is read as "sigma." To calculate a standard deviation, the rule $\sigma = \sqrt{ \dfrac{ (x_1 - \bar{x})^2 + (x_2 - \bar{x})^2 + \ldots + (x_n - \bar{x})^2}{n} }$ is used, where $n$ is the number of values in the data set and $\bar{x}$ is the mean of the set.