Use a normal effects plot to compare the relative magnitude and the statistical significance of both main and interaction effects. Minitab draws a line to indicate where the points would be expected to fall if all effects were zero. Points that do not fall near the line usually signal significant effects. Such effects are larger and generally further from the fitted line than unimportant effects. By default, Minitab uses an a-level of 0.05 and labels any effect that is significant.

For the insulation data, there are two significant effects (a = 0.05):

·    One main effect - material type (A)

·    One interaction effect - material type by injection pressure (AB)