A modified intervention model for gross domestic product variable (agriculture)
An intervention analysis of agricultural product was conducted. To obtain stationarity, a regular and seasonal differencing was applied to the series. Intervention point of the series was identified in the year 1981. Thus, the pre-intervention period was modeled using the Box and Jenkins (1970) ARIMA methods. Using Akaike Information Criteria (AIC), the pre and post -intervention periods were identified to be Seasonal ARIMA (0,1,0)(1,0,0)4 and Seasonal ARIMA (0,1,1)(2,0,0)2 . On the application of the usual Transfer Function Method, the intervention model was found to be inadequate. To obtain adequacy, a modified approach involving variable reduction was used. The new approach was found to give adequate fit to the series. The model was then used to generate forecast for the next 24 time points.
Keywords: Statinarity, Seasonal autoregressive integrated moving average models, Intervention model and Transfer function