On Wed, 25 Feb 2009, Heather Turner wrote:
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
So let's be clear: this was the 'offset' argument' and not the
offset() function as you refer to below.
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.
Not ignored, just not yet resolved so nothing really useful to say as
yet. I suspect the documentation was once correct but the offset
argument had some rather undesirable prperties at the time. And there
appears to be some legacy code to do the recycling.
So quite a bit of work is needed on the history to deal wth what is
rather a small point: sorting out some problems with failing packages
(rgdal and friends) has been a much higher priority (let alone the day
jobs).
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.
That used not to be the case for the offset _argument_, so probably
where the docs came from.
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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Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
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