Refer to the data of Exercise 12.13. The experiment was also concerned with the effects of high levels of copper in the chick feed. Five of the 10 chicks in each level of the feed additive received 400 ppm of copper, while the remaining five chicks received no copper. The data are given here.

Let y be the feed efficiency ratio, x1 be the amount of the feed additive, and x2 be the amount of copper placed in the feed. Fit the following two models:

a. Which of the two models appears to provide the better fit to the data? Justify your ­answer.
b. Display the predicted equation for the best- fitting model.
c. Explain the meaning of 1 in the best- fitting model.
In exercise

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Copper Level Additive Feed Eflidiency Ratio 1.30, 135, 144 152.156 し61, L48,156, L45, L14 2.17, 2.11,2.08, 2.13,2.22 2.29.2.33,224, 2.16, 2.21 2.30,2.34,2.20, 2.38,2.48 2.44,2.37 243, 2.37 2.41 2.47, 2.51,2.79, 240,2.55 2.67, 2.50, 2.55, 2.60,2.49 3.31.317 3.24. 321,3.35 3.38, 342, 336, 3.25,3.51 4.92.387 481, 4.88,5.06 5.09,497 4.95, 4.59,4.76 0 0 20 20 0 60 0 400 100 100 400 Additive Feed Efficiency Ratio (gm Feed to gm WtGain) 0 20 40 60 80 100 1.30,1.35, 144, 152, 1.56, 161, 148, 156, 145, 1.14 217 2.11.2.08,2.13, 222,2.29,2.33, 2.24, 2.16,221 2.30,2.34,2.20,2.38, 2.48,2.44, 2.37 2.43,2.32 2.41 247 2.51.2.79,240.2.55,267 250,2.55,2.60,2.49 3.31,3.17 3.24, 321,3.35,3.38, 342,3.36,3.25,351 4.92.3.87 4.81,488, 5.06,5.09.497 4.95, 4.59,4.76