[Rd] aliased coefficients in summary.glm

[John Fox]

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