16.28 In Figure 16.10, fitted autoregressive models of orders 1 through 4 are given for annual sales data. We then selected a model by testing the null hypothesis of autoregression of order p - 1 against the alternative of autoregression of order p at the 5% significance level. Repeat this procedure, but test at the 10% significance level.

a. What autoregressive model is now selected?

b. Obtain forecasts of sales for the next 3 years, based on this selected model.