# SARIMAX_CHECK - Check Parameters' Values for Model Stability

Examines the model's parameters for stability constraints (e.g., stationary, invertibility, causality, etc.).

## Syntax

SARIMAX_CHECK (µ, σ, d, [φ], [θ], d, sd, [sφ], [sθ], [β])

µ
Optional. Is the ARMA model long-run mean (i.e., mu). If missing, the process mean is assumed to be zero.
σ
Required. Is the standard deviation value of the model's residuals/innovations.
D
Required. Is the non-seasonal integration order.
[φ]
Optional. Are the parameters of the non-seasonal AR(p) component model: [φ1, φ2 … φp] (starting with the lowest lag).
[θ]
Optional. Are the parameters of the MA(q) component model: [θ1, θ2 … θq] (starting with the lowest lag).
S
Optional. Is the number of observations per period (e.g., 12 = Annual, 4 = Quarter).
sD
Optional. Is the seasonal integration order.
[sφ]
Optional. Are the parameters of the seasonal AR(P) component model: [sφ1, sφ2 … sφpp] (starting with the lowest lag).
[sθ]
Optional. Are the parameters of the seasonal MA(Q) component model: [sθ1, sθ2 … sθqq] (starting with the lowest lag).
[β]
Optional. Is the coefficients array of the exogenous factors.

## Remarks

1. The underlying model is described here.
2. The time series is homogeneous or equally spaced.
3. SARIMA_CHECK checks if $\sigma\gt 0$ and if all the characteristic roots of the underlying ARMA model fall outside the unit circle.
4. Using the Solver Add-in in Excel, you can specify the return value of SARIMA_CHECK as a constraint to ensure a stationary ARMA model.
5. The intercept or the regression constant term input argument is optional. If omitted, a zero value is assumed.
6. For the input argument - ([β]):
• The input argument is optional and can be omitted, in which case no regression component is included (i.e., plain SARIMA).
• The order of the parameters defines how the exogenous factor input arguments are passed.
• One or more parameters may have a missing value or an error code (i.e., #NUM!, #VALUE!, etc.).
7. The long-run mean argument (µ) can take any value or be omitted, in which case a zero value is assumed.
8. The residuals/innovations standard deviation - (σ) - must be greater than zero.
9. For the input argument - ([φ]) (parameters of the non-seasonal AR component):
• The input argument is optional and can be omitted, in which case no non-seasonal AR component is included.
• The order of the parameters starts with the lowest lag.
• One or more parameters may have missing values or error codes (i.e., #NUM!, #VALUE!, etc.).
• The order of the non-seasonal AR component model is solely determined by the order of the last value in the array with a numeric value (vs. missing or error).
10. For the input argument - ([θ]) (parameters of the non-seasonal MA component):
• The input argument is optional and can be omitted, in which case no non-seasonal MA component is included.
• The order of the parameters starts with the lowest lag.
• One or more values in the input argument can be missing or an error code (i.e., #NUM!, #VALUE!, etc.).
• The order of the non-seasonal MA component model is solely determined by the order of the last value in the array with a numeric value (vs. missing or error).
11. For the input argument - ([sφ]) (parameters of the seasonal AR component):
• The input argument is optional and can be omitted, in which case no seasonal AR component is included.
• The order of the parameters starts with the lowest lag.
• One or more parameters may have missing values or error codes (i.e., #NUM!, #VALUE!, etc.).
• The order of the seasonal AR component model is solely determined by the order of the last value in the array with a numeric value (vs. missing or error).
12. For the input argument - ([sθ]) (parameters of the seasonal MA component):
• The input argument is optional and can be omitted, in which case no seasonal MA component is included.
• The order of the parameters starts with the lowest lag.
• One or more values in the input argument can be missing or an error code (i.e., #NUM!, #VALUE!, etc.).
• The order of the seasonal MA component model is solely determined by the order of the last value in the array with a numeric value (vs. missing or error).
13. The non-seasonal integration order - (d) - is optional and can be omitted, in which case d is assumed to be zero.
14. The seasonal integration order - (sD) - is optional and can be omitted, in which case sD is assumed to be zero.
15. The season length - (s) - is optional and can be omitted, in which case s is assumed to be zero (i.e., plain ARIMA).
16. The function was added in version 1.63 SHAMROCK.