A simple linear regression model is employed to analyze the 24 monthly observations given in Table 15.10 on the next page. Residuals are computed and are plotted versus time. The resulting residual plot is shown in Figure 15.37 on the next page. Discuss why the residual plot suggests the existence of positive autocorrelation. The Durbin-Watson statistic d can be calculated to be .473. Test for positive (first-order) autocorrelation at α = .05, and test for negative (first-order) autocorrelation at α = .05.