From Glenn.Stone@csiro.au  Thu Oct 24 05:33:05 2002
From: Glenn.Stone@csiro.au (Glenn.Stone@csiro.au)
Date: Thu, 24 Oct 2002 06:33:05 +0200 (MET DST)
Subject: model.matrix (via predict) (PR#2206)
Message-ID: <200210240433.GAA24513@pubhealth.ku.dk>

Full_Name: Glenn Stone
Version: 1.5.1 and 1.6.0
OS: win2000
Submission from: (NULL) (168.140.227.9)


The following code produces incorrect fitted values in version 1.5.1 and an
error in 1.6.0 

   Error in "contrasts<-"(*tmp*, value = "contr.treatment") : 
           contrasts apply only to factors
   In addition: Warning message: 
   variable ihalf is not a factor in: model.frame.default(object, data, xlev =
xlev) 

The problem in 1.5.1 seems to be model.matrix return inconsistently ordered
interaction terms. There are abviously several work arounds.


### Start of example
dfr <- expand.grid(i=0:154,j=0:154)
dfr <- dfr[with(dfr,i+j<=154),]

dfr$logj <- with(dfr,log(j+0.5))
dfr$jmin6 <- with(dfr, pmin(j,6))
dfr$ihalf <- with(dfr, factor(floor(i/6)))
dfr$i100 <- with(dfr, ifelse(i<100,1,0))
dfr$j0 <- with(dfr, ifelse(j==0,1,0))

Ai <- runif(length(levels(dfr$ihalf)))
Ai <- Ai-Ai[1]
Bi <- runif(length(levels(dfr$ihalf)))
Bi <- (Bi-Bi[1])/10

eta <- with(dfr, -5 -2.5*logj - 3*i100*j0
            + 0.01*jmin6+Ai[as.numeric(ihalf)]
            +Bi[as.numeric(ihalf)]*jmin6)

dfr$clms <- sapply( eta , function(x) rpois(1,1e6*exp(x)))
                                            
gg <- glm(clms~ logj + i100:j0 + jmin6 + ihalf + ihalf:jmin6,
          data=dfr,
          family=poisson)

plot(predict(gg),predict(gg,newdata=dfr))
### End of example

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model.matrix (via predict) (PR#2206)
Glenn.Stone@csiro.au