[R] RE: Ordinal data - Regression Trees & Proportional Odds

[Prof Brian D Ripley]

On Mon, 2 Jun 2003, Andreas Christmann wrote:

>  >>> 1. RE: Ordinal data - Regression Trees  & Proportional Odds
>        (Liaw, Andy)
>
>  > AFAIK there's no implementation (or description) of tree algorithm
>  > that handles ordinal response.
>  >
>
> Regression trees with an ordinal response variable can be computed with
> SPSS Answer Tree 3.0.

They *can* be handled by tree or rpart in R.

I think Andy's point was that there is no consensus as to the right way to
handle them:  certainly using the codes of categories works and may often
be reasonable, and treating ordinal responses as categorical is also very
often perfectly adequate.

Note that rpart is user-extensible, so it would be reasonably easy to write
an extension for a proportional-odds logistic regression model, if that is
thought appropriate (and it seems strange to me to impose such strong
structure on the model with such a general `linear predictor': POLR
models are often in my experience a poor reflection of real problems).

-- 
Brian D. Ripley,                  ripley at 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 272860 (secr)
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