Refer to the Business Week magazine data in Problem 13 of Chapter 8. The relationships among the yield (Y), 1989 rank (X2), and P-E ratio (X3) were studied in that problem using a random sample of data from the magazine's compilation of information on the top 1,000 companies. The results of the regression of yield on 1989 rank, P-E ratio, and 1990 rank (X1) are presented next. Use this output and the output from Problem 13 in Chapter 8 to answer the following questions.
a. Test whether 1989 rank, P-E ratio, and 1990 rank, taken together, significantly help to predict the yield.
b. Test whether 1990 rank is associated with the yield after taking into account the contribution of 1989 rank and P-E ratio.
c. Should 1990 rank be included in the model to control for confounding? Explain your answer, including discussion of any assumptions you made in reaching your answer.