## 9.2 Regression with ARIMA errors in R

The R function `Arima()`

will fit a regression model with ARIMA errors if the argument `xreg`

is used. The `order`

argument specifies the order of the ARIMA error model. If differencing is specified, then the differencing is applied to all variables in the regression model before the model is estimated. For example, the R command

will fit the model y_t' = \beta_1 x'_t + \eta'_t, where \eta'_t = \phi_1 \eta'_{t-1} + \varepsilon_t is an AR(1) error. This is equivalent to the model
y_t = \beta_0 + \beta_1 x_t + \eta_t,
where \eta_t is an ARIMA(1,1,0) error. Notice that the constant term disappears due to the differencing. To include a constant in the differenced model, specify `include.drift=TRUE`

.

The `auto.arima()`

function will also handle regression terms via the `xreg`

argument. The user must specify the predictor variables to include, but `auto.arima()`

will select the best ARIMA model for the errors. If differencing is required, then all variables are differenced during the estimation process, although the final model will be expressed in terms of the original variables.

The AICc is calculated for the final model, and this value can be used to determine the best predictors. That is, the procedure should be repeated for all subsets of predictors to be considered, and the model with the lowest AICc value selected.

### Example: US Personal Consumption and Income

Figure 9.1 shows the quarterly changes in personal consumption expenditure and personal disposable income from 1970 to 2016 Q3. We would like to forecast changes in expenditure based on changes in income. A change in income does not necessarily translate to an instant change in consumption (e.g., after the loss of a job, it may take a few months for expenses to be reduced to allow for the new circumstances). However, we will ignore this complexity in this example and try to measure the instantaneous effect of the average change of income on the average change of consumption expenditure.

```
autoplot(uschange[,1:2], facets=TRUE) +
xlab("Year") + ylab("") +
ggtitle("Quarterly changes in US consumption
and personal income")
```

```
(fit <- auto.arima(uschange[,"Consumption"],
xreg=uschange[,"Income"]))
#> Series: uschange[, "Consumption"]
#> Regression with ARIMA(1,0,2) errors
#>
#> Coefficients:
#> ar1 ma1 ma2 intercept xreg
#> 0.692 -0.576 0.198 0.599 0.203
#> s.e. 0.116 0.130 0.076 0.088 0.046
#>
#> sigma^2 = 0.322: log likelihood = -156.9
#> AIC=325.9 AICc=326.4 BIC=345.3
```

The data are clearly already stationary (as we are considering percentage changes rather than raw expenditure and income), so there is no need for any differencing. The fitted model is \begin{align*} y_t &= 0.599 + 0.203 x_t + \eta_t, \\ \eta_t &= 0.692 \eta_{t-1} + \varepsilon_t -0.576 \varepsilon_{t-1} + 0.198 \varepsilon_{t-2},\\ \varepsilon_t &\sim \text{NID}(0,0.322). \end{align*}

We can recover estimates of both the \eta_t and \varepsilon_t series using the `residuals()`

function.

```
cbind("Regression Errors" = residuals(fit, type="regression"),
"ARIMA errors" = residuals(fit, type="innovation")) %>%
autoplot(facets=TRUE)
```

It is the ARIMA errors that should resemble a white noise series.

```
#>
#> Ljung-Box test
#>
#> data: Residuals from Regression with ARIMA(1,0,2) errors
#> Q* = 5.89, df = 5, p-value = 0.32
#>
#> Model df: 3. Total lags used: 8
```