The standard deviation formula is very simple: it is the square root of the variance. It is the most commonly used measure of **spread**.

An important attribute of the standard deviation as a measure of spread is that if the mean and standard deviation of a **normal distribution** are known, it is possible to compute the percentile rank associated with any given score. In a normal distribution, about 68% of the scores are within one standard deviation of the mean and about 95% of the scores are within two standard deviations of the mean.

The standard deviation has proven to be an extremely useful measure of spread in part because it is mathematically tractable. Many formulas in inferential statistics use the standard deviation.

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