Normal Probability Plot of the Effects
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In the normal probability plot of the effects, points that do not fall near the line usually indicate important effects. Important effects are larger and generally further from the fitted line than unimportant effects. Unimportant effects tend to be smaller and centered around zero.

If there is no error term, Minitab uses Lenth's method [5] to identify important effects. If there is an error term, Minitab uses the corresponding p-values that are shown in the Session window to identify important effects. The normal probability plot uses a = 0.05, by default. You can change the a-level by changing the confidence level in the Options subdialog box or by setting a in the Stepwise subdialog box.

This plot shows that terms A, B,  and AB are significant and are in the model. Term C is not significant but is in the model. Terms AC, BC, and ABC are not in the model.