6. An ARIMA(1, 1, 0) model (AR(1) model for first differences) is fit to observations of a time series. The first 12 residual autocorrelations are shown in Figure P-6. The model was fit with a constant.

a. Based on an inspection of the residual autocorrelations, does the model appear to be adequate? Why or why not?

b. If you decide the model is not adequate, indicate how it might be modified to eliminate any inadequacy.