Use the ANOM main effects plot to test the hypothesis that each factor level mean is equal to the overall mean at an a-level you specify. Minitab displays one main effect plot for each factor in a two-way design. The main effects plot shows the following:

·    Plotted points – the sample means at each factor level.

·    Center line (green) – the overall mean.

·    Lower and upper decision limits (red) – used to test the hypothesis. Minitab looks for sample means located beyond the decision limits and marks them with a red symbol.

-    If a sample mean is located beyond a decision limit, you can reject the hypothesis that the mean is equal to the overall mean.

-    If a sample mean falls within the decision limits, you cannot reject the hypothesis that the mean is equal to the overall mean.

The driving data contains two factors, Driving Experience and RoadType. The main effects plots indicate the following:

·    The overall mean is 12.917, which is the center line in both graphs.

·    The sample means for both levels of Experience are beyond the decision limits. Therefore, you can reject the hypothesis that the means for Experience are equal to the overall mean. The means for both Experience 1 and Experience 2 differ from the overall mean.

·    The sample means for two levels of RoadType are beyond the decision limits. Therefore, you can reject the hypothesis that the means for RoadType are all equal to the overall mean. In fact, the means for RoadType 1 and RoadType 3 differ from the overall mean.