Dirt bikes. Off-road motorcycles (often called “dirt bikes”) are a segment (about 18%) of the growing motorcycle market. Because dirt bikes offer great variation in features, they are a good market segment to study to learn about which features account for the cost (manufacturer’s suggested retail price, MSRP) of a bike. Researchers collected data on 2005-model dirt bikes (lib.stat.cmu.edu/

datasets/dirtbike_aug.csv). Their original goal was to study market differentiation among brands. (The Dirt on Bikes:

An Illustration of CART Models for Brand Differentiation, Jiang Lu, Joseph B. Kadane, and Peter Boatwright). In Chapter 18, Exercises 41, 42, and 43 dealt with these data, but several bikes were removed from those data to simplify the analysis. Now we’ll take on the full set.13 Here’s a regression model and some associated graphs.

a) List aspects of this regression model that lead to the conclusion that it is likely to be a useful model.

b) What aspects of the displays indicate that the model is a good one?

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Dependent variable is: MSRP R-squared 91.0% R-squared (adjusted) = 90.5% s = 606.4 with 100 694 degrees of freedom Source Sum of Squares df Mean Square F-ratio Regression 349911096 190 5 69982219 Residual 34566886 94 367733 Variable Coeff SE(Coeff) t-ratio P-value Intercept -5514.66 826.2 -6.67 0.0001 Bore 83.7950 6.145 13.6 0.0001 Clearance 152.617 52.02 2.93 0.0042 Engine Strokes -315.812 89.83 -3.52 0.0007 Total Weight -13.8502 3.017 -4.59 0.0001 Wheelbase 119.138 34.26 3.48 0.0008