In order to find a confidence interval using the z-distribution, first we need a data set. The file cholest.mtw in the Minitab data folder has four columns of data, which reflect the results of an experiment involving cholesterol reduction programs. In this experiment a number of subjects had their cholesterol measured following the completion of a two, four, or fourteen-day program. The data was also collected for a control group. Our interest will be whether or not there is any difference in the mean levels of the four groups.
Cholesterol levels seem to be normally distributed.
Thus, even though our sample sizes are small, the procedure for finding confidence intervals with the normal distribution is appropriate. It is believed that the population standard deviation for cholesterol levels is approximately 50. Open the file cholest.mtw. In the worksheet you will see 4 variables. At this point a dialog box appears in which you select various options.