The main effects plot is most useful when you have several factors. You can then compare the changes in the level means to see which factors influence the response the most. A main effect is present when different levels of a factor affect the response differently. For a factor with two levels, you may find that one level increases the mean compared to the other level. This difference is a main effect.

Minitab creates the main effects plot by plotting the response mean for each factor level. A line connects the points for each factor. Minitab also draws a reference line at the overall mean. Look at the line to determine whether or not a main effect is present for a factor.

·    When the line is horizontal (parallel to the x-axis), then there is no main effect present. Each level of the factor affects the response in the same way, and the response mean is the same across all factor levels.

·    When the line is not horizontal (parallel to the x-axis), then there is a main effect present. Different levels of the factor affect the response differently. The greater the difference in the vertical position of the plotted points (the more the line is not parallel to the X-axis), the greater the magnitude of the main effect.

By comparing the slopes of the lines, you can compare the relative magnitude of the factor effects.

Although a table of means and a plot of means provide the same numerical information, you may find it easier to judge a plot than a table of numbers. As always, plots indicate patterns. To determine if a pattern is statistically significant, you must do an appropriate test.

For the driving data, the plots indicate the following:

·    Experience: Experienced drivers made fewer corrections than inexperienced drivers.

·    RoadType: Dirt roads required more corrections than second class roads. Both dirt roads and second class roads required more corrections than first class roads.

·    The overall mean (12.917) is plotted with a black line across each panel.

These plots do not show any dramatic difference in the magnitude of the effect on the response between the factors Experience and RoadType. Both factors seem to affect the response by approximately the same amount. The response changes with Experience from approximately 16 to 10, with Road Type from about 9 to 12.5 to 17.5.