[R] extract data from lm object and then use again?

[Thomas Lumley]

On Fri, 22 Sep 2006, Thilo Kellermann wrote:

> Hi,
>
> the data of the model fit is stored in lm$model and should work....
>

Not reliably. In the first place, you should use the accessor function 
model.frame(model) rather than model$model, which works even if the model 
was fitted with model=FALSE.

But even then,
   glm(formula(model), data=model.frame(model))
will not work reliably.
Consider
   model <- lm(log(Volume)~log(Height)+log(Girth),data=trees)

The model frame has variables called eg "log(Volume)" rather than 
"Volume".

When you need the source data frame you need to do something like
    eval(model$call$data, environment(formula(model)))
and even this might not work, eg if the model had no data 
argument.

However, if the model had no data argument then the variables 
must be available in environment(formula(model)), in which case any data 
frame of the right size will do.

If there are no missing observations or the model  was fitted with 
na.action="na.exclude" then a fairly reliable approach is to use
   eval(model$call$data, environment(formula(model)))
if it is not NULL and to fall back to model.frame(model). This is what 
termplot() does.


 	-thomas