The Intervention Component: X( It)

Chapter 4 introduces the full ARIMA intervention model. Most substantive theories specify the intervention as an exogenous dichotomy. A Box-Tiao transfer function then distributes the intervention's response across the endogenous time series to reflect a theoretically specified onset and duration. Transfer functions allow the noise component to be parsed from the residualized time series. Theoretical specification of the intervention model requires at least some sense of the onset and duration of the impact. Detailed analyses of ten time series demonstrate how to handle interventions with abrupt and permanent, gradually accruing, gradually decaying, and complex impacts. One popular version of an ITSA short course ends with Chapter 4. Although statistically adequate ARIMA models can be built using the modeling strategy described in Chapters 3-4, survey knowledge of the auxiliary methods described in Chapter 5 is recommended.