R devel - Apr 2003 - aliased coefficients in summary.glm

Dear list members,

I was reminded of what I see as a problem with summary.glm this week when 
some of my students fell into a trap in a homework exercise, defining a 
logit model in which coefficients were aliased. When this happens in lm, 
summary.lm prints a message ["Coefficients: (1 not defined because of 
singularities)"], but summary.glm is silent. In both instances, the print 
methods show aliased coefficients as NA.

At minimum, I think that it would be desirable for summary.glm to indicate 
that there's a problem. In addition, I would prefer that both summary.glm 
and summary.lm include aliased coefficients (as NA) in the table of 
coefficients.

Thanks for your attention,
  John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox@mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

John,

This has only just arrived.  It is not a good time to be raising such 
issues (1.7.0 is frozen, so this would have to be for 1.8.0).  Could you 
send it as a wishlist item to R-bugs?  Otherwise it is very likely to be 
forgotten about.

Brian

On Thu, 10 Apr 2003, John Fox wrote:
> Dear list members,
> 
> I was reminded of what I see as a problem with summary.glm this week when 
> some of my students fell into a trap in a homework exercise, defining a 
> logit model in which coefficients were aliased. When this happens in lm, 
> summary.lm prints a message ["Coefficients: (1 not defined because of 
> singularities)"], but summary.glm is silent. In both instances, the
print
> methods show aliased coefficients as NA.
> 
> At minimum, I think that it would be desirable for summary.glm to indicate 
> that there's a problem. In addition, I would prefer that both
summary.glm
> and summary.lm include aliased coefficients (as NA) in the table of 
> coefficients.
-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595