18. Although the time series data in Table P-18 are old, they provide the basis for some interesting regression modeling. Using the data in Table P-18, attempt to relate personal savings to personal income (in billions of dollars) for the time period from 1935 to 1954.

a. Fit a simple linear regression model to the data in Table P-18, using personal income to predict personal savings. (1) Test for the significance of the regression at the level; (2) calculate , and interpret this quantity; and (3) test for autocorrelation using the Durbin-Watson test with . Should you modify your conclusion in part 1? How can the model be improved?

b. Construct a dummy variable, , for the WWII years. Let for peacetime and for wartime.The war years are from 1941 to 1945. Fit a multiple linear regression model using personal income and the war years dummy variable as predictor variables for personal savings. Evaluate the results. Specifically (1) test to determine whether knowledge of the war years makes a significant contribution to the prediction of personal savings beyond that provided by personal income (set ) and (2) test for autocorrelation. Is the multiple regression model better than the simple linear regression model of part a? Discuss.