Re: [Rd] [R] length 1 offset in glm (& lm)

From: Heather Turner <Heather.Turner_at_warwick.ac.uk>
Date: Wed, 25 Feb 2009 12:48:27 +0000


This post about length 1 offsets on R help seems to have been ignored (sorry deleted original email - is there a way to continue thread in this case?)

https://stat.ethz.ch/pipermail/r-help/2009-February/189352.html

It does seem to be a bug, in that glm does not behave as documented. In fact the same bug applies to lm as well.

I don't think the suggested fix works though - Y isn't available until the model frame is evaluated. Moreover you would want to evaluate the offset as part of the model frame, to take care of the search path, subset and na.action.

One possibility would be to modify model.frame to recycle variables of length one. This would be a potentially useful feature from my point of view as offset(constant) could replace Const(constant) in gnm, which is basically a work around for this problem in the case of specifying a constant in a nonlinear term. However, this solution may have undesired side-effects and I don't feel too strongly about it as there are various  work-arounds. Perhaps the simplest solution would be to modify the docs so that offsets of length one are disallowed - it is easy enough for the user to create a vector of the right length as necessary.

Best regards,

Heather



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