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 (see Chapter 9). Consequently, the fourth differenced data will be Y5 €“ Y1 = 19.64 - .17 = 19.47, Y6 - Y2 = 19.24 - 15.13 = 4.11,..., and so forth.
b. Plot the time series of fourth differences. Does this time series appear to be stationary or nonstationary? Explain.
Problem 23
Table P-23 contains Southwest Airlines' quarterly income before extraordinary items ($MM) for the years 1988-1999.

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TABLE P-23 Quarterly Income for Southwest Airlines (SMM) 2nd 3rd 26.59 24.57 23.04 15.72 26.89 48.83 58.62 67.72 60.86 92.51 129.64 126.98 Year 1st 4th 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 0.17 19.64 5.09 8.21 13.48 24.93 41.85 11.83 33.00 50.87 70.01 95.85 15.13 19.24 23.53 10.57 23.48 42.15 58.52 59.72 85.32 93.83 33.39 157.76 16.07 8.11 4.58 8.84 27.17 38.37 20.34 43.36 28.16 80.55 100.38 93.80