Interval plots contain several key elements:

·    The mean on the y-axis.

·    Groups based on categorical grouping variables along the x-axis.

·    The mean value represented by a circle.

·    The interval represented by a vertical line with horizontal lines at the upper and lower limits. By default, Minitab displays individual 95% confidence intervals around the mean.

Use interval plots to display the sample mean along with a range of likely values for the population mean.

This interval plot shows the amount of torque applied at different temperatures before the chucks spun out of the log. Although the means appear to be different, the difference is probably not significant because all the interval bars easily overlap:

·    The mean torque applied at 60 degrees is 26.1, and the 95% confidence interval extends from 17.9 to 34.4.

·    The mean torque applied at 120 degrees is 24.3, and the 95% confidence interval extends from 17.4 to 31.1.

·    The mean torque applied at 150 degrees is 21.6, and the 95% confidence interval extends from 15.5 to 27.7.