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 (Jiang Lu, Joseph B. Kadane, and Peter Boatwright, The Dirt on Bikes: An Illustration of CART Models for Brand Differentiation). We’ve updated their data to 2014-model bikes. In Chapter 18, Exercises 41, 42, and 43 dealt with these data.
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?