An experiment to test a new gasoline additive, Gasplus, was performed on three different cars: a sports car, a minivan, and a hybrid. Each car was tested with both Gasplus and regular gas on 10 different occasions, and their gas mileage was recorded. Here are the boxplots.
A two-way ANOVA with interaction model was run, and the following ANOVA table resulted.
A plot of the residuals vs. predicted values showed:
What conclusions about the additive and car types do you draw? Do you see any potential problems with the analysis?
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