A testing laboratory studies and compares the relationship between tire tread wear per 1,000 miles (Y) and average driving speed (X) for two competing tire types (denoted A and B). The data shown in the following table were collected for a random sample of 20 tires, 10 of A and 10 of B.

a. Identify a single regression model that uses the data for both tire types and that defines straight-line models relating tread wear (Y) to average speed (X) for each tire.
b. Using the computer output given next, determines and plot the fitted straight lines for each tire type.
c. Test the null hypothesis that the straight lines for the two tire types coincide.
d. Test H0: "The lines are parallel" versus HA: "The lines are not parallel."
e. In light of your answers to parts (c) and (d), comment on differences and similarities in the tread wear-average speed relationship for the two tire types.
Regression of Tread Wear (Y) on Avg. Speed (X), Tire (Z), and XZ
(Z = 0 for Tire A, Z = 1 for Tire B)
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