Re: [R] ARIMA xreg and factors

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Tue 16 Jan 2007 - 07:50:56 GMT

>From the help page

```     xreg: Optionally, a vector or matrix of external regressors, which
must have the same number of rows as 'x'.

```

Note, not a factor. (It handles your factor like the integer vector it is internally.) If you want a design matrix you need to make one, something like

include.mean = FALSE, xreg = model.matrix(~z)

On Mon, 15 Jan 2007, sj wrote:

> I am using arima to develop a time series regression model, I am using arima
> b/c I have autocorrelated errors. Several of my independent variables are
> categorical and I have coded them as factors . When I run ARIMA I don't
> get any warning or error message, but I do not seem to get estimates for all
> the levels of the factor. Can/how does ARIMA handle factors in xreg?
>
>
> here is some example code:
>
> sim.ar <- arima.sim(list(ar = c(0.4, 0.4)), n = 1000)
> z<- factor(rep((1:5),200))
>
> arima(sim.ar,order=c(2,0,0),xreg=z)
>
>
> I only get a single estiamte for xreg. Am I thinking about this wrong (I
> expected 4).
>
> thank you,
>
>
> Spencer
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
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>

```--
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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```
Received on Tue Jan 16 18:55:01 2007

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