Several extensions to autoregressive integrated moving average (ARIMA) models have been considered in the recent years. Many of them are special cases of the extended ARIMA model treated in this paper. The main features are time-dependent coefficients in the autoregressive and moving average polynomials, various types of interventions (including the usual Box and Tiao form and the innovation interventions but also interventions acting on the scale), trends on the level or on the scale, built-in deterministic seasonal components and variable transformations. In the past, estimation procedures were limited to least squares although the evaluation of the likelihood function is available for special cases, including the ARMA model with time dependent coefficients. This paper deals with maximum likelihood estimation when several features of the extended ARIMA model are taken together.
INTRODUCTIONSeveral extensions of ARIMA models have been considered in the recent years, including (a) the use of time-dependent coefficients in the autoregressive and moving average polynomials (Quenouille, 1957;Whittle, 1965;Abdrabbo and Priestley, 1967;Miller, 1968 and1969;Subba Rao, 1970; Mélard and Kiehm, 1981;Tyssedal and Tjøstheim, 1982;Grillenzoni, 1990), (b) various types of interventions, including the usual Box and Tiao (1975) formulation and the innovational interventions (Fox, 1972) but also interventions acting on the scale (Mélard, 1981a;Tsay, 1988), (c) additive (level) or multiplicative (scale) trend (Mélard, 1977), (d) built-in deterministic seasonal components on the variable (Abraham and Box, 1978) or on the innovation (Mélard, 1981b), (e) variable transformations (Box and Cox, 1964). 1 We are grateful to the referee for his/her comments, and more especially for the summary that we have used at the end of Section 1.
2The purpose of that model is to encompass several deterministic variations with respect to time in the framework of the usual stochastic ARIMA models. Other extensions not explicitly considered in this paper are ARMA models with GARCH errors (Bollerslev, 1986), threshold AR models (Tong, 1983), bilinear models (Subba Rao, 1981), fractional differencing ARIMA models (Granger and Joyeux, 1980). It should be noted, however, that some of these extensions can be handled using the same approach. For instance, threshold ARMA models (Mélard and Roy, 1988) can be seen as time-dependent ARMA models. Other approaches for time-dependent models include spectral density estimation (Priestley 1981(Priestley , 1988, recursive estimation (Ljung and Söderström, 1983;Young, 1984) and models with random coefficients (Nicholls and Quinn, 1982;Bougerol, 1992).Motivations for the extended ARIMA model which is used here have already been discussed elsewhere (Mélard, 1982a(Mélard, , 1985a). An illustration has already been provided (Mélard, 1985b). The estimation procedure was however limited to the conditional least squares approach, generalizing the approach of Box and Jenkins (1976). In this paper, an algorithm f...