For Exercises 2.44 and 2.45, see page 82; for 2.46, see page 84; for 2.47, see page 86; for 2.48 and 2.49, see page 88; for 2.50, see page 90; for 2.51, see page 90; for 2.52, see page 91; and for 2.53, see page 94.

Question 2.63

2.63 Fuel efficiency and CO2 emissions

Refer to the previous exercise.

canfuel

  1. Make a scatterplot of the data with highway MPG as the response variable and city MPG as the explanatory variable. Include the least-squares regression line on the plot. There is an unusual pattern for the vehicles with high city MPG. Describe it.
  2. Make a plot of the residuals versus city MPG. Describe the major features of this plot. How does the unusual pattern noted in part (a) appear in this plot?
  3. The Lexus 350h AWD that you examined in parts (b) and (c) of the previous exercise is in the group of unusual cases mentioned in parts (a) and (b) of this exercise. It is a hybrid vehicle that uses a conventional engine and a electric motor that is powered by a battery that can recharge when the vehicle is driven. The conventional engine also turns off when the vehicle is stopped in traffic. As a result of these features, hybrid vehicles are unusually efficient for city driving, but they do not have a similar advantage when driven at higher speeds on the highway. How do these facts explain the residual for this vehicle?
  4. Several Toyota vehicles are also hybrids. Use the residuals to suggest which vehicles are in this category.

2.63

(a) The vehicles with high city MPG don't follow the regression line; rather, they have a much lower highway MPG than the regression line would predict. (b) For the vehicles with high city MPG, all of the residuals are negative, creating a curve in the plot, suggesting a possible transformation is necessary. (c) Because the hybrid vehicles have an electric motor in addition to the conventional motor, which is intended to improve city MPG, we would expect them to have a much better city MPG than expected, which is why their residuals fall so far below the residuals for the conventional motor vehicles. (d) Three Toyota Prius models and two Toyota Camry Hybrid models likely are hybrids.