Using the data for Problem 16.15 on page 646 that represent U.S. total computer and software sales (in $millions) from 1992 through 2012 (stored in Computer Sales),
a. Fit a third-order autoregressive model to the total sales and test for the significance of the third-order autoregressive parameter. (Use a = 0.05.)
b. If necessary, fit a second-order autoregressive model to the total sales and test for the significance of the second-order autoregressive parameter. (Use a = 0.05.)
c. If necessary, fit a first-order autoregressive model to the total sales and test for the significance of the first-order autoregressive parameter. (Use a = 0.05.)
d. If appropriate, forecast the total sales in 2013?