Minitab uses two methods of calculating a confidence interval for the standard deviation and variance: the chi-square method and the Bonett method. Use these guidelines to determine which method to use:
- If your sample data come from a normally distributed population, use the chi-square method. However, this method will produce inaccurate results if your sample does not come from an approximately normal population.
- The Bonett method is robust against departures from normality. If your data are strongly nonnormal, the Bonett method is significantly more accurate than the chi-square method. Minitab cannot calculate the Bonett method with summarized data.