Standard deviation

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The average of a set of numbers' difference from the mean. The difference from the mean for each piece of data is squared (the number subtract the mean or x - x-bar), found the average and square rooted. After this it is the standard deviation.

It is squared and square rooted afterwards to remove the effect of any negative values from subtracting the piece of data from the mean. If the number is not square rooted at the end it is called the variance. This is why the standard deviation is often called the square root of the variance.