Consider the modeling of oil well drilling costs described in Problem 13.2.3 and the data set in DS 13.2.3. Suppose that a model is used with cost as the dependent variable and with depth and downtime as input variables,
(a) Make plots of the residuals against the tilled values and against each of the two input variables. What do you find?
(b) Make a plot of the residuals against the variable geology. Why does this plot confirm that the variable geology is not needed in the regression model?
(c) Verify that there are no points with an unusually large influence on the regression model.