[R] Automatically Remove Aliased Terms from a Model

[Eik Vettorazzi]

Hi Thorn,
it is not entirely clear (at least for me) what you want to accomplish.
an easy and fail safe way of extracting used terms in a (g)lm-object is
names(model.frame(l))
if you want to extract terms to finally select a model, have a look at
drop1 and/or MASS::dropterm

Hth

Am 28.10.2013 17:19, schrieb Thaler,Thorn,LAUSANNE,Applied Mathematics:
> Dear all,
> 
> I am trying to implement a function which removes aliased terms from a model. The challenge I am facing is that with "alias" I get the aliased coefficients of the model, which I have to translate into the terms from the model formula. What I have tried so far:
> 
> ------------------8<------------------
> d <- expand.grid(a = 0:1, b=0:1)
> d$c <- (d$a + d$b)  %% 2
> d$y <- rnorm(4)
> d <- within(d, {a <- factor(a); b <- factor(b); c <- factor(c)})
> l <- lm(y ~ a * b + c, d)
> 
> removeAliased <- function(mod) {
>   ## Retrieve all terms in the model
>   X <- attr(mod$terms, "term.label")
>   ## Get the aliased coefficients  
>   rn <- rownames(alias(mod)$Complete)
>   ## remove factor levels from coefficient names to retrieve the terms
>   regex.base <- unique(unlist(lapply(mod$model[, sapply(mod$model, is.factor)], levels)))
>   aliased <- gsub(paste(regex.base, "$", sep = "", collapse = "|"),  "", gsub(paste(regex.base, ":", sep = "", collapse = "|"), ":", rn))
>   uF <- formula(paste(". ~ .", paste(aliased, collapse = "-"), sep = "-"))
>   update(mod, uF)
> }
> 
> removeAliased(l)
> ------------------>8------------------
> 
> This function works in principle, but this workaround with removing the factor levels is just, well, a workaround which could cause problems in some circumstances (when the name of a level matches the end of another variable, when I use a different contrast and R names the coefficients differently etc. - and I am not sure which other cases I am overlooking).
> 
> So my question is whether there are some more intelligent ways of doing what I want to achieve? Is there a function to translate a coefficient of a LM back to the term, something like:
> 
> termFromCoef("a1") ## a1
> termFromCoef("a1:b1") ## a:b
> 
> With this I could simply translate the rownames from alias into the terms needed for the model update.
> 
> Thanks for your help.
> 
> Kind Regards,
> 
> Thorn Thaler 
> NRC Lausanne
> Applied Mathematics
> 
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> and provide commented, minimal, self-contained, reproducible code.
> 

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Eik Vettorazzi

Department of Medical Biometry and Epidemiology
University Medical Center Hamburg-Eppendorf

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