Refer to Exercise 12.23. Fit a complete second- order model relating y to x1, x2, x3, and x4. That is, include both first- and second- order terms in the four variables along with all six cross-product terms, xi xj.
a. Compare the R2 values for the three models. Which model appears to provide the best fit?
b. Test at the .05 level whether any of the cross- product terms provide a significant relationship with y, maximal oxygen uptake.
c. Which of the three models would you recommend? Justify your answer.