From ripley@stats.ox.ac.uk  Tue Dec  5 07:51:16 2000
From: ripley@stats.ox.ac.uk (Prof Brian D Ripley)
Date: Tue, 5 Dec 2000 07:51:16 +0000 (GMT)
Subject: [Rd] Inconsistency, possibly a bug? (PR#758)
In-Reply-To: <200012050026.BAA10701@pubhealth.ku.dk>
Message-ID: <Pine.GSO.4.05.10012050747500.21711-100000@auk.stats>

On Tue, 5 Dec 2000 presnell@stat.ufl.edu wrote:

> 
> Seems to be a day for finding peculiar little things.  There is an
> inconsistency in the behavior of lm vis a vis glm:
> 
>  > x <- rnorm(15)
>  > y <- 1 + 10*x + rnorm(15)
>  > z <- as.factor(rep(c("A","B","C"),rep(5,3)))
>  > xyz <- data.frame(x,y,z)
>  > fit.lm <- lm(y ~ x + z, data=xyz, subset=(z != "C"))
>  > fit.glm <- glm(y ~ x + z, family=gaussian, data=xyz, subset=(z != "C"))
>  > zz <- z[1:10]
>  > xx <- rnorm(10)
>  > predict(fit.lm,data.frame(x=xx,z=zz))
>  Error in model.frame.default(formula, data, xlev = xlev) : 
>  	  factor z has new level(s) C
>  > predict(fit.glm,data.frame(x=xx,z=zz))
>  	   1          2          3          4          5          6 
>    2.570970 -18.007372  18.108771  12.498562   9.566029  10.518460 
>  	   7          8          9         10 
>   -9.132206  11.440242 -16.054621  -3.017842 
> 
> 
> This happens because of the line
> 
>   mf$drop.unused.levels <- TRUE
> 
> in lm and the lack of same in glm.  I'm reporting this as a bug, but
> perhaps the difference is intentional?

No, as the glm code is not protected from user error as the lm code is. I
am not sure it is bug (it's not documented to happen) and probably there
are several related occurrences in other modelling functions, but it would
be a desirable addition and I will change it for glm (at least).


> 
> --please do not edit the information below--
> 
> Version:
>  platform = sparc-sun-solaris2.6
>  arch = sparc
>  os = solaris2.6
>  system = sparc, solaris2.6
>  status = 
>  major = 1
>  minor = 1.1
>  year = 2000
>  month = August
>  day = 15
>  language = R
> 
> Search Path:
>  .GlobalEnv, Autoloads, package:base
> 
> -- 
> Brett Presnell
> Department of Statistics
> University of Florida
> 
> 
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-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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[Rd] Inconsistency, possibly a bug? (PR#758)
Prof Brian D Ripley