[R] Avoiding singular fits in rlm

[Peter Ehlers]

Matt Crawford wrote:
> I keep coming back to this problem of singular fits in rlm (MASS library),
> but cannot figure out a good solution.
> 
> I am fitting a linear model with a factor variable, like
> lm( Y ~ factorVar)
> and this works fine. lm knows to construct the contrast matrix the way I
> would expect, which puts the first factor as the baseline level.
> 
> But when I try
> rlm( Y ~ factorVar)
> I get the message "'x' is singular: singular fits are not implemented in
> rlm."

Could it be that some level(s) of factorVar have no corresponding
Y-values? Check with table().

  -Peter Ehlers

> 
> How can I solve this problem so that I can use factor variables in rlm?
> 
> I've tried constructing the contrast matrix myself using the various "contr"
> functions, but I always run out of memory. So I must not be doing something
> right, because lm has to create some version of the same contrast matrix in
> memory. In any case, I don't think that more memory is the solution.
> 
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