The stack loss dataset (Brownlee, 1965, p. 454) given in Table 5.12 are observations from 21 days’ operation of a plant for the oxidation of ammonia, a stage in the production of nitric acid. The predictors X1, X2, X3 are respectively the air flow, the cooling water inlet temperature, and the acid concentration, and the response Y is the stack loss. Fit a linear model to these data using the LS estimator, the MM-estimators with efficiencies 0.95 and 0.80, and the DCML estimator, and compare the results. Fit the residuals against the day. Is there a pattern?