[This exercise requires a computer program to carry out the multiple regression calcula tions.] Refer to the data of Exercise 30, showing earnings over 24 months. Denote the ob servations X. (1, 2,.... 24). Now, form the series of first differences (12.3.....24) Fit autoregressive models of orders 1 through 4 to the series Z. Using the approach of Section 17.9 for testing that the autoregressive order is (p-1) against the alternative of order p. with a 10% significance level, select one of these models. Using the selected model, find forecasts for Z., t 25, 26, 27.

Hence, obtain forecasts of earnings for the next 3 months