Gas additives. 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.

What conclusions about the additive and car types do you draw? Do you see any potential problems with the analysis?

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50.0 37.5 25.0 12.5 Bdu pasipy HTH 0 6dw pasnipy Hybrid Minivan Sports Type HOH HH $ Gasplus Regular Additive A two-way ANOVA with interaction model was run, and the following ANOVA table resulted. Sum of Mean Source DF Squares Square F-ratio P-value 2 23175.4 11587.7 2712.2 <0.0001 Additive Interaction 2 1 92.1568 92.1568 21.57 <0.0001 51.8976 25.9488 6.0736 0.0042 Error Total 54 230.711 4.27242 59 23550.2 A plot of the residuals vs. predicted values showed: 25 Residuals (mpg) 0 -2.5 -5.0+ +++ +++++ + + + + + 50.0 12.5 25.0 37.5 Predicted Values (mpg)