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# arima class

Superclasses:

Create ARIMA or ARIMAX time series model

## Description

`arima` creates model objects for stationary or unit root nonstationary linear time series model. This includes moving average (MA), autoregressive (AR), mixed autoregressive and moving average (ARMA), integrated (ARIMA), multiplicative seasonal, and linear time series models that include a regression component (ARIMAX).

Specify models with known coefficients, estimate coefficients with data using `estimate`, or simulate models with `simulate`. By default, the variance of the innovations is a positive scalar, but you can specify any supported conditional variance model, such as a GARCH model.

## Construction

`Mdl = arima` creates an ARIMA model of degrees zero.

```Mdl = arima(p,D,q)``` creates a nonseasonal linear time series model using autoregressive degree `p`, differencing degree `D`, and moving average degree `q`.

`Mdl = arima(Name,Value)` creates a linear time series model using additional options specified by one or more `Name,Value` pair arguments. `Name` is the property name and `Value` is the corresponding value. `Name` must appear inside single quotes (`''`). You can specify several name-value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

### Note

You can only use these arguments for nonseasonal models. For seasonal models, use the name-value syntax.

 `p` Positive integer indicating the degree of the nonseasonal autoregressive polynomial. `D` Nonnegative integer indicating the degree of nonseasonal integration in the linear time series. `q` Positive integer indicating the degree of the nonseasonal moving average polynomial.

#### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside quotes. You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

 `'AR'` Cell vector of nonseasonal autoregressive coefficients corresponding to a stable polynomial. When specified without `ARLags`, `AR` is a cell vector of coefficients at lags 1,2,... to the degree of the nonseasonal autoregressive polynomial. When specified with `ARLags`, `AR` is an equivalent-length cell vector of coefficients associated with the lags in `ARLags`. Default: Cell vector of `NaN`s. `'ARLags'` Vector of positive integer lags associated with the `AR` coefficients. Default: Vector of integers 1,2,... to the degree of the nonseasonal autoregressive polynomial. `'Beta'` Real vector of coefficients corresponding to the regression component in an ARIMAX conditional mean model. Default: `[]` (no regression coefficients corresponding to a regression component) `'Constant'` Scalar constant in the linear time series. Default: `NaN` `'D'` Nonnegative integer indicating the degree of the nonseasonal differencing lag operator polynomial (the degree of nonseasonal integration) in the linear time series. Default: `0` (no nonseasonal integration) `'Distribution'` Conditional probability distribution of the innovation process. `Distribution` is `'Gaussian'` or `'t'`. Alternatively, specify it as a data structure with the field `Name` to store the distribution `'Gaussian'` or `'t'`. If the distribution is `'t'`, then the structure also needs the field `DoF` to store the degrees of freedom. Default: `'Gaussian'` `'MA'` Cell vector of nonseasonal moving average coefficients corresponding to an invertible polynomial. When specified without `MALags`, `MA` is a cell vector of coefficients at lags 1,2,... to the degree of the nonseasonal moving average polynomial. When specified with `MALags`, `MA` is an equivalent-length cell vector of coefficients associated with the lags in `MALags`. Default: Cell vector of `NaN` values. `'MALags'` Vector of positive integer lags associated with the `MA` coefficients. Default: Vector of integers 1,2,... to the degree of the nonseasonal moving average polynomial. `'SAR'` Cell vector of seasonal autoregressive coefficients corresponding to a stable polynomial. When specified without `SARLags`, `SAR` is a cell vector of coefficients at lags 1,2,... to the degree of the seasonal autoregressive polynomial. When specified with `SARLags`, `SAR` is an equivalent-length cell vector of coefficients associated with the lags in `SARLags`. Default: Cell vector of `NaN`s. `'SARLags'` Vector of positive integer lags associated with the `SAR` coefficients. Default: Vector of integers 1,2,... to the degree of the seasonal autoregressive polynomial. `'SMA'` Cell vector of seasonal moving average coefficients corresponding to an invertible polynomial. When specified without `SMALags`, `SMA` is a cell vector of coefficients at lags 1,2,... to the degree of the seasonal moving average polynomial. When specified with `SMALags`, `SMA` is an equivalent-length cell vector of coefficients associated with the lags in `SMALags`. Default: Cell vector of `NaN`s. `'SMALags'` Vector of positive integer lags associated with the `SMA` coefficients. Default: Vector of integers 1,2,... to the degree of the seasonal moving average polynomial. `'Seasonality'` Nonnegative integer indicating the degree of the seasonal differencing lag operator polynomial in the linear time series model. Default: `0` (no seasonal integration) `'Variance'` Positive scalar variance of the model innovations, or a supported conditional variance model object (e.g., a `garch` model object). Default: `NaN` `'Description'` String scalar or character vector describing the model. By default, this argument describes the parametric form of the model, for example, ```"ARIMA(1,1,1) Model (Gaussian Distribution)"```.

### Notes

• Each `AR`, `SAR`, `MA`, and `SMA` coefficient is associated with an underlying lag operator polynomial and is subject to a near-zero tolerance exclusion test. That is, the software compares each coefficient to the default lag operator zero tolerance, `1e-12`. If the magnitude of a coefficient is greater than `1e-12`, then the software includes it in the model. Otherwise, the software considers the coefficient sufficiently close to 0, and excludes it from the model. For additional details, see `LagOp`.

• Specify the lags associated with the seasonal polynomials `SAR` and `SMA` in the periodicity of the observed data, and not as multiples of the `Seasonality` parameter. This convention does not conform to standard Box and Jenkins  notation, but it is a more flexible approach for incorporating multiplicative seasonality.

## Properties

 `AR` Cell vector of nonseasonal autoregressive coefficients corresponding to a stable polynomial. Associated lags are 1,2,... to the degree of the nonseasonal autoregressive polynomial, or as specified in `ARLags`. `Beta` Real vector of regression coefficients corresponding to a regression component. `Constant` Scalar constant in the linear time series model. `D` Nonnegative integer indicating the degree of nonseasonal integration in the linear time series. `Description` String scalar for the model description. `Distribution` Data structure for the conditional probability distribution of the innovation process. The field `Name` stores the distribution name `"Gaussian"` or `"t"`. If the distribution is `"t"`, then the structure also has the field `DoF` to store the degrees of freedom. `MA` Cell vector of nonseasonal moving average coefficients corresponding to an invertible polynomial. Associated lags are 1,2,... to the degree of the nonseasonal moving average polynomial, or as specified in `MALags`. `P` Degree of the compound autoregressive polynomial. `P` is the total number of lagged observations necessary to initialize the autoregressive component of the model. `P` includes the effects of nonseasonal and seasonal integration captured by the properties `D` and `Seasonality`, respectively, and the nonseasonal and seasonal autoregressive polynomials `AR` and `SAR`, respectively. The property `P` does not necessarily conform to standard Box and Jenkins notation. It only conforms if the model has no integration nor seasonal autoregressive component. `Q` Degree of the compound moving average polynomial. `Q` is the total number of lagged innovations necessary to initialize the moving average component of the model. `Q` includes the effects of nonseasonal and seasonal moving average polynomials `MA` and `SMA`, respectively. The property `Q` does not necessarily conform to standard Box and Jenkins notation. It only conforms if the model has no seasonal moving average component. `SAR` Cell vector of seasonal autoregressive coefficients corresponding to a stable polynomial. Associated lags are 1,2,... to the degree of the seasonal autoregressive polynomial, or as specified in `SARLags`. `SMA` Cell vector of seasonal moving average coefficients corresponding to an invertible polynomial. Associated lags are 1,2,... to the degree of the seasonal moving average polynomial, or as specified in `SMALags`. `Seasonality` Nonnegative integer indicating the seasonal differencing polynomial degree in the linear time series model. `Variance` Positive scalar variance of the model innovations, or a supported conditional variance model (e.g., a `garch` model).

## Methods

 estimate Estimate ARIMA or ARIMAX model parameters filter Filter disturbances using ARIMA or ARIMAX model forecast Forecast ARIMA or ARIMAX model responses or conditional variances impulse Impulse response function infer Infer ARIMA or ARIMAX model residuals or conditional variances print (To be removed) Display parameter estimation results for ARIMA or ARIMAX models simulate Monte Carlo simulation of ARIMA or ARIMAX models summarize Display ARIMA model estimation results

## Copy Semantics

Value. To learn how value classes affect copy operations, see Copying Objects (MATLAB).

## Examples

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Specify an ARIMA(2,1,2) model,

`$\left(1-{\varphi }_{1}L-{\varphi }_{2}{L}^{2}\right)\left(1-L\right){y}_{t}=\left(1+{\theta }_{1}L+{\theta }_{2}{L}^{2}\right){\epsilon }_{t}$`

`Mdl = arima(2,1,2)`
```Mdl = arima with properties: Description: "ARIMA(2,1,2) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 3 D: 1 Q: 2 Constant: NaN AR: {NaN NaN} at lags [1 2] SAR: {} MA: {NaN NaN} at lags [1 2] SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN ```

The model is nonseasonal, so you can use shorthand syntax. The result is a model with two nonseasonal AR coefficients ($p$ = 2), two nonseasonal MA coefficients ($q$ = 2), and one degree of differencing ($D$ = 1). The property `P` is equal to $p$ + $D$ = 3. `NaN` values indicate estimable parameters.

Create, and then modify an `arima` model.

Specify an AR(3) model with known coefficients,

`${y}_{t}=0.05+0.6{y}_{t-1}+0.2{y}_{t-2}-0.1{y}_{t-3}+{\epsilon }_{t},$`

where ${\epsilon }_{t}$ has a Gaussian distribution with mean 0 and variance 0.01.

```Mdl = arima('Constant',0.05,'AR',{0.6,0.2,-0.1},... 'Variance',0.01)```
```Mdl = arima with properties: Description: "ARIMA(3,0,0) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 3 D: 0 Q: 0 Constant: 0.05 AR: {0.6 0.2 -0.1} at lags [1 2 3] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: 0.01 ```

Modify the model object to make all the model parameters unknown (set them to `NaN`).

```Mdl.Constant = NaN; Mdl.AR = {NaN NaN NaN}; Mdl.Variance = NaN```
```Mdl = arima with properties: Description: "ARIMA(3,0,0) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 3 D: 0 Q: 0 Constant: NaN AR: {NaN NaN NaN} at lags [1 2 3] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN ```

Make the innovation distribution a $t$ distribution with 10 degrees of freedom.

```tdist = struct('Name','t','DoF',10); Mdl.Distribution = tdist```
```Mdl = arima with properties: Description: "ARIMA(3,0,0) Model (t Distribution)" Distribution: Name = "t", DoF = 10 P: 3 D: 0 Q: 0 Constant: NaN AR: {NaN NaN NaN} at lags [1 2 3] SAR: {} MA: {} SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN ```

Specify an MA model with no constant, and moving average terms at lags 1, 2, and 12,

`${y}_{t}={\epsilon }_{t}+{\theta }_{1}{\epsilon }_{t-1}+{\theta }_{2}{\epsilon }_{t-2}+{\theta }_{12}{\epsilon }_{t-12}.$`

`Mdl = arima('Constant',0,'MALags',[1,2,12])`
```Mdl = arima with properties: Description: "ARIMA(0,0,12) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 0 D: 0 Q: 12 Constant: 0 AR: {} SAR: {} MA: {NaN NaN NaN} at lags [1 2 12] SMA: {} Seasonality: 0 Beta: [1×0] Variance: NaN ```

Specify a multiplicative seasonal ARIMA model with seasonal and nonseasonal integration,

`$\left(1-L\right)\left(1-{L}^{12}\right){y}_{t}=\left(1+{\theta }_{1}\right)\left(1+{\theta }_{12}{L}^{12}\right){\epsilon }_{t}.$`

```Mdl = arima('Constant',0,'D',1,'Seasonality',12,... 'MALags',1,'SMALags',12)```
```Mdl = arima with properties: Description: "ARIMA(0,1,1) Model Seasonally Integrated with Seasonal MA(12) (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 13 D: 1 Q: 13 Constant: 0 AR: {} SAR: {} MA: {NaN} at lag  SMA: {NaN} at lag  Seasonality: 12 Beta: [1×0] Variance: NaN ```

An ARIMA model is assumed to be multiplicative any time `SMALags` or `SARLags` are specified.

Specify an ARIMAX model with one or more regression coefficients corresponding to predictor data.

Specify the ARIMAX(1,1,1) model,

`$\left(1-0.2L\right)\left(1-L{\right)}^{1}{y}_{t}=0.5{x}_{t}+\left(1+0.3L\right){\epsilon }_{t}$`

using `arima`.

`Mdl = arima('AR',0.2,'D',1,'MA',0.3,'Beta',0.5)`
```Mdl = arima with properties: Description: "ARIMAX(1,1,1) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 2 D: 1 Q: 1 Constant: NaN AR: {0.2} at lag  SAR: {} MA: {0.3} at lag  SMA: {} Seasonality: 0 Beta: [0.5] Variance: NaN ```

In the output, the property `P` is sum of the AR lags and degree of nonseasonal integration `p + D = 2`.

Modify this ARIMAX(1,1,1) model by adding two more regression coefficients,

` Mdl.Beta=[0.5,4,-0.6]`
```Mdl = arima with properties: Description: "ARIMAX(1,1,1) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 2 D: 1 Q: 1 Constant: NaN AR: {0.2} at lag  SAR: {} MA: {0.3} at lag  SMA: {} Seasonality: 0 Beta: [0.5 4 -0.6] Variance: NaN ```

Specify an ARIMA(1,0,1) conditional mean model with a GARCH(1,1) conditional variance model.

Specify the conditional mean model.

`Mdl = arima(1,0,1);`

Specify the conditional variance model.

`Mdl.Variance = garch(1,1)`
```Mdl = arima with properties: Description: "ARIMA(1,0,1) Model (Gaussian Distribution)" Distribution: Name = "Gaussian" P: 1 D: 0 Q: 1 Constant: NaN AR: {NaN} at lag  SAR: {} MA: {NaN} at lag  SMA: {} Seasonality: 0 Beta: [1×0] Variance: [GARCH(1,1) Model] ```

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