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This example shows how to use the shorthand `arima(p,D,q)`

syntax to specify the default AR() model,

By default, all parameters in the created model object have unknown values, and the innovation distribution is Gaussian with constant variance.

Specify the default AR(2) model:

model = arima(2,0,0)

model = arima with properties: Description: "ARIMA(2,0,0) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 2 D: 0 Q: 0 Constant: NaN AR: {NaN NaN} at lags [1 2] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN

The output shows that the created model object, `model`

, has `NaN`

values for all model parameters: the constant term, the AR coefficients, and the variance. You can modify the created model object using dot notation, or input it (along with data) to `estimate`

.

This example shows how to specify an AR(*p*) model with constant term equal to zero. Use name-value syntax to specify a model that differs from the default model.

Specify an AR(2) model with no constant term,

where the innovation distribution is Gaussian with constant variance.

model = arima('ARLags',1:2,'Constant',0)

model = arima with properties: Description: "ARIMA(2,0,0) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 2 D: 0 Q: 0 Constant: 0 AR: {NaN NaN} at lags [1 2] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN

The `ARLags`

name-value argument specifies the lags corresponding to nonzero AR coefficients. The property `Constant`

in the created model object is equal to `0`

, as specified. The model object has default values for all other properties, including `NaN`

values as placeholders for the unknown parameters: the AR coefficients and scalar variance.

You can modify the created model object using dot notation, or input it (along with data) to `estimate`

.

This example shows how to specify an AR(*p*) model with nonzero coefficients at nonconsecutive lags.

Specify an AR(4) model with nonzero AR coefficients at lags 1 and 4 (and no constant term),

where the innovation distribution is Gaussian with constant variance.

model = arima('ARLags',[1,4],'Constant',0)

model = arima with properties: Description: "ARIMA(4,0,0) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 4 D: 0 Q: 0 Constant: 0 AR: {NaN NaN} at lags [1 4] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN

The output shows the nonzero AR coefficients at lags 1 and 4, as specified. The property `P`

is equal to `4`

, the number of presample observations needed to initialize the AR model. The unconstrained parameters are equal to `NaN`

.

Display the value of `AR`

:

model.AR

`ans = `*1x4 cell array*
{[NaN]} {[0]} {[0]} {[NaN]}

The `AR`

cell array returns four elements. The first and last elements (corresponding to lags 1 and 4) have value `NaN`

, indicating these coefficients are nonzero and need to be estimated or otherwise specified by the user. `arima`

sets the coefficients at interim lags equal to zero to maintain consistency with MATLAB® cell array indexing.

This example shows how to specify an ARMA(*p*, *q*) model with known parameter values. You can use such a fully specified model as an input to `simulate`

or `forecast`

.

Specify the ARMA(1,1) model

where the innovation distribution is Student's *t* with 8 degrees of freedom, and constant variance 0.15.

tdist = struct('Name','t','DoF',8); model = arima('Constant',0.3,'AR',0.7,'MA',0.4,... 'Distribution',tdist,'Variance',0.15)

model = arima with properties: Description: "ARIMA(1,0,1) Model (t Distribution)" Distribution: Name = "t", DoF = 8 P: 1 D: 0 Q: 1 Constant: 0.3 AR: {0.7} at lag [1] SAR: {} MA: {0.4} at lag [1] SMA: {} Seasonality: 0 Beta: [1×0] Variance: 0.15

Because all parameter values are specified, the created model has no `NaN`

values. The functions `simulate`

and `forecast`

don't accept input models with `NaN`

values.

This example shows how to specify an AR() model with a Student's *t* innovation distribution.

Specify an AR(2) model with no constant term,

where the innovations follow a Student's *t* distribution with unknown degrees of freedom.

model = arima('Constant',0,'ARLags',1:2,'Distribution','t')

model = arima with properties: Description: "ARIMA(2,0,0) Model (t Distribution)" Distribution: Name = "t", DoF = NaN P: 2 D: 0 Q: 0 Constant: 0 AR: {NaN NaN} at lags [1 2] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN

The value of `Distribution`

is a `struct`

array with field `Name`

equal to `'t'`

and field `DoF`

equal to `NaN`

. The `NaN`

value indicates the degrees of freedom are unknown, and need to be estimated using `estimate`

or otherwise specified by the user.

In the **Econometric
Modeler** app, you can specify the lag structure, presence of a constant,
and innovation distribution of an AR(*p*) model by following these
steps. All specified coefficients are unknown, estimable parameters.

In the

**Data Browser**, select the response time series to which the model will be fit.On the

**Econometric Modeler**tab, in the**Models**section, click**AR**.The

**AR Model Parameters**dialog box appears.Specify the lag structure. To specify an AR(

*p*) model that includes all AR lags from 1 through*p*, use the**Lag Order**tab. For the flexibility to specify the inclusion of particular lags, use the**Lag Vector**tab. For more details, see Specifying Lag Operator Polynomials Interactively. Regardless of the tab you use, you can verify the model form by inspecting the equation in the**Model Equation**section.

For example:

To specify an AR(2) model that includes a constant, includes the first lag, and has a Gaussian innovation distribution, in the

**Autoregressive Order**box, type`2`

.To specify an AR(2) model that includes the first lag, has a Gaussian distribution, but does not include a constant:

In the

**Autoregressive Order**box, type`2`

.Clear the

**Include Constant Term**box.

To specify the AR(4) model containing nonconsecutive lags

$${y}_{t}={\varphi}_{1}{y}_{t-1}+{\varphi}_{4}{y}_{t-4}+{\epsilon}_{t},$$

where

*ε*is a series of IID Gaussian innovations,_{t}Click the

**Lag Vector**tab.In the

**Autoregressive Lags**box, type`1 4`

.Clear the

**Include Constant Term**box.

To specify an AR(2) model that includes the first lag and a constant term, and has a

*t*innovations distribution:In the

**Autoregressive Lags**box, type`2`

.Click

**Innovation Distribution**>`t`

.

The degrees of freedom parameter of the

*t*distribution is an unknown, but estimable parameter.

After you specify a model, click **Estimate** to
estimate all unknown parameters.

- Econometric Modeler Overview
- Specify Conditional Mean Models
- Modify Properties of Conditional Mean Model Objects
- Specify Conditional Mean Model Innovation Distribution

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