Definition:ARIMA Model/ARIMA Operator
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Definition
Let $S$ be a stochastic process based on an equispaced time series.
Let the values of $S$ at timestamps $t, t - 1, t - 2, \dotsc$ be $z_t, z_{t - 1}, z_{t - 2}, \dotsc$
Let $a_t, a_{t - 1}, a_{t - 2}, \dotsc$ be a sequence of independent shocks at timestamps $t, t - 1, t - 2, \dotsc$
Let:
- $w_t = \nabla^d z_t$
where $\nabla^d$ denotes the $d$th iteration of the backward difference operator.
Let $M$ be an ARIMA process on $S$:
- $\tilde w_t = \phi_1 w_{t - 1} + \phi_2 w_{t - 2} + \dotsb + \phi_p w_{t - p} + a_t - \theta_1 a_{t - 1} - \theta_2 a_{t - 2} - \dotsb - \theta_q a_{t - q}$
Using the autoregressive operator:
- $\map \phi B = 1 - \phi_1 B - \phi_2 B^2 - \dotsb - \phi_p B^p$
and the moving average operator:
- $\map \theta B = 1 - \theta_1 B - \theta_2 B^2 - \dotsb - \theta_q B^q$
the ARIMA model can be written in the following compact manner:
- $\map \phi B w_t = \map \theta B a_t$
where $B$ denotes the backward shift operator.
Hence:
- $\map \varphi B z_t = \map \phi B \paren {1 - B}^d z_t = \map \theta B a_t$
where:
- $\map \varphi B = \map \phi B \paren {1 - B}^d$
In practice, $d$ is usually $0$ or $1$, or at most $2$.
Sources
- 1994: George E.P. Box, Gwilym M. Jenkins and Gregory C. Reinsel: Time Series Analysis: Forecasting and Control (3rd ed.) ... (previous) ... (next):
- $1$: Introduction:
- $1.2$ Stochastic and Deterministic Dynamic Mathematical Models
- $1.2.1$ Stationary and Nonstationary Stochastic Models for Forecasting and Control: Nonstationary models
- $1.2$ Stochastic and Deterministic Dynamic Mathematical Models
- $1$: Introduction: