[R] Using xlevels

[Prof Brian Ripley]

On Wed, 30 Mar 2011, Terry Therneau wrote:

> I'm working on predict.survreg and am confused about xlevels.
> The model.frame method has the argument, but none of the standard
> methods (model.frame.lm, model.frame.glm) appear to make use of it.

But I see this in predict.lm:

         m <- model.frame(Terms, newdata, na.action = na.action,
                          xlev = object$xlevels)

It is used to remap levels in newdata to those used in the fit.

>
> The documentation for model.matrix states:
>  xlev: to be used as argument of model.frame if data has no "terms"
> attribute.

Well, the code says

     if (is.null(attr(data, "terms")))
         data <- model.frame(object, data, xlev=xlev)

> But the terms attribute has no xlevels information in it, so I find this
> statement completely confusing.  Any insight is appreciated.

It means exactly what it says: a 'data' argument with a terms 
attribute is considered to be a model frame.

>
> Terry Therneau
>
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-- 
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)
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