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

[Prof Brian Ripley]

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,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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