Mr. Ng is a trader in the commodity market. He has followed the corn price for a few years and would like to use an ARIMA model for forecasting. The time series plot of the price (in cents) per bushel of corn for 120 consecutive months is shown in Figure 2-1. The autocorrelation plot of the corn price is shown in Figure 2-2. After some transformation, the sample autocorrelation function and sample partial autocorrelation function of the transformed data are shown in Figure 2-3 and Figure 2-4. 24 72 84 Month Figure 2-1: Time series plot of the corn price 6 810 12 14 20 2224 26 28 12 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -1.0 2 4 120 30 Figure 2-2: Autocorrelation function for the corn price