6.11.1. Draper and Smith (1966, p. 41) considered data on the relationship between the age of truck tractors (in years) and the cost (in dollars) of maintaining them over a six-month period. The data are given in Table 6.8. Plot cost versus age and fit a regression of cost on age. Give 95% confidence intervals for the slope and intercept. Give a 99% confidence interval for the mean cost of maintaining tractors that are 2.5 years old. Give a 99% prediction interval for the cost of maintaining a particular tractor that is 2.5 years old.

Reviewing the plot of the data, how much faith should be placed in these estimates for tractors that are 2.5 years old?