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Standard Deviation

A statistical term that expresses the variability in a data set. It is indicated by the Greek letter sigma, and calculated in 3 steps: - Estimate the average (mean) of the data. - Find the difference between each data and the mean, and then square each difference. - Sum all squared differences. - Divide the sum by the number of data minus 1. - Calculate the square root of the above. 68% of the data will fall within one standard deviation of the mean. For a standard deviation of 2, this value goes up to 95%, whereas for 3 the standard deviation is 99.7%.