Probe a little deeper into the analysis in Exercise 49 as follows: 

a) Plot the data and the residuals, calculate the Durbin-Watson statistic and comment on what these imply for the regression model.

b) Would a transformation of the data improve the model: (i) for 2000–08; (ii) for 2009–12; (iii) for 2013–15? You are not asked to perform the transformation; if you think a transformation might help, indicate which transformation to use.