[R] interaction terms formula error

[Frank E Harrell Jr]

Alejandra Solis Herrera wrote:
> Hi,
> 
>  
> 
> I'm trying to fit a binary logistic regression model, and would like to consider certain characteristics B and C only for people with variable A=1 and not for those with variable A=0, so I'm trying to do the following:
> 
>  
> 
> model<- lrm(formula= y ~  A: (B+C) + D + E +...)
> 
>  
> 
> I've had no problem with adding interaction terms using " * ", but every time I try to use " : ", as in my example, I get an error message like:
> 

The Design package doesn't support that.  It uses the (highly 
recommended) hierarchy principle so it assumes you will use * for 
interactions.

Frank

>  
> 
> Error in if (!length(fname) || !any(fname == zname)) { : 
> 
>             Missing value where TRUE/FALSE needed
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>  
> 
> I would really appreciate any advice on how to solve this, I don't know what in the notation should be different for different operators.
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>  
> 
> Thanks in advance, kind regards, 
> 
>  
> 
> Alejandra Solís 
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>  
> 
> 
> 	[[alternative HTML version deleted]]
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> 
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-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University