From presnell@stat.ufl.edu  Tue Dec  5 00:26:51 2000
From: presnell@stat.ufl.edu (presnell@stat.ufl.edu)
Date: Tue, 5 Dec 2000 01:26:51 +0100 (MET)
Subject: [Rd] Inconsistency, possibly a bug? (PR#758)
Message-ID: <200012050026.BAA10701@pubhealth.ku.dk>


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?

--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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[Rd] Inconsistency, possibly a bug? (PR#758)
presnell@stat.ufl.edu