Example 2.7

EXAMPLE 2.7 Fuel Efficiency and CO₂ Emissions

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Let's look at the relationship between highway MPG and CO₂ emissions, two quantitative variables, while also taking into account the type of fuel, a categorical variable. The JMP statistical software was used to produce the plot in Figure 2.6. We see that there is a negative relationship between the two quantitative variables. Better (higher) MPG is associated with lower CO₂ emissions. The relationship is curved, however, not linear.

Figure 2.6: FIGURE 2.6 Scatterplot of CO₂ emissions versus highway MPG for 1067 vehicles for the model year 2014 using JMP software. Colors correspond to the type of fuel used: blue for diesel, red for ethanol, green for regular gasoline, and purple for premium gasoline, Example 2.7.

The legend on the right side of the figure identifies the colors used to plot the four types of fuel, our categorical variable. The vehicles that use regular gasoline (green) and premium gasoline (purple) appear to be mixed together. The diesel-burning vehicles (blue) are close to the the gasoline-burning vehicles, but they tend to have higher values for both MPG and emissions. On the other hand, the vehicles that burn ethanol (red) are clearly separated from the other vehicles.