Table P-23 contains Southwest Airlines' quarterly income before extraordinary items ($MM) for the years 1988-1999.

a. Plot the income data as a time series and describe any patterns that exist.
b. Is this series stationary or nonstationary? Explain.
c. Using Minitab or a similar program, compute the autocorrelations of the income series for the first 10 time lags. Is the behavior of the autocorrelations consistent with your choice in part b? Explain.

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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