[R] Baseline terms for lrm

[David Winsemius]

On Jan 18, 2011, at 2:51 PM, Rob James wrote:

> Dear R-help and Prof. Harrell:
>
> My question concerns the baseline state for continuous variable in  
> lrm() within the RMS package.
>
> I have a model which can be reduced to:
>
> lrm(FT ~ rcs(V1, c(0, 1,5))
>
> The model makes perfect sense if the baseline state is where V1>=5  
> but the model makes no sense if the baseline category is 0 (which I  
> had expected).

You are using terminology that suggests you ar working with a  
categorical variable but asking a question about a function that only  
makes sense when applied to contiguous variables. I think you need to  
provide results from str and describe() on V1.

> Can someone point me to a reference, or suggest a command/test, to  
> get the model to fess up to the baseline state. The variable V1 and  
> V1' are shown in the model output, but there is no clear association  
> between these pseudo-variables and the ranges specified in the cut.

Further suggestive evidence that you are working with categorical data  
if there was construction of the variable with cut() function.

> Thus, it is seems possible to misidentify the baseline value for the  
> term.
>
> Many thanks,

David Winsemius, MD
West Hartford, CT