In Mean plots, size Error Bars using: Use this parameter to specify the dispersion statistic to use. Options are listed in the following table: Select: When you want to: Standard Deviation Use the standard deviation of the mean . Standard Error of the Mean Divide the standard deviation by the square root of ( n - 1), where n is the sample size . To Specify the Dispersion Statistic: Click the radio button corresponding to the statistic that you want to use.