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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