24. This question refers to Problem 23.

a. Use Minitab or Excel to compute the fourth differences of the income data in Table P-23. Fourth differences are computed by differencing observations four time periods apart. With quarterly data, this procedure is sometimes useful for creating a stationary series from a nonstationary series. Consequently, the fourth differenced data will be .

b. Plot the time series of fourth differences. Does this time series appear to be stationary or nonstationary? Explain.