Forgive my resending this post. To data I have received only one response
(thank you Bert Gunter), and I still do not have an answer to my question.
Respectfully,
John
Windows XP
R 2.12.1
contrast package.
I am trying to understand how to create contrasts for a model that contatains
an interaction. I can get contrasts to work for a model without interaction,
but not after adding the interaction. Please see code below. The last two
contrast statements show the problem. I would appreciate someone letting me
know what is wrong with the syntax of my contrast statements.
Thank you,
John
library(contrast)
# Create 2x2 contingency table.
counts=c(50,50,30,70)
row <- gl(2,2,4)
column <- gl(2,1,4)
mydata <- data.frame(row,column,counts)
print(mydata)
# Show levels of 2x2 table
levels(mydata$row)
levels(mydata$column)
# Models, no interaction, and interaction
fitglm0 <- glm(counts ~ row + column, family=poisson(link="log"))
fitglm <- glm(counts ~ row + column + row*column, family=poisson(link="log"))
# Contrasts for model without interaction works fine!
anova(fitglm0)
summary(fitglm0)
con0<-contrast(fitglm0,list(row="1",column="1"))
print(con0,X=TRUE)
# Contrast for model with interaction does not work.
anova(fitglm)
summary(fitglm)
con<-contrast(fitglm,list(row="1",column="1")
print(con,X=TRUE)
# Nor does this work.
con<-contrast(fitglm,list(row="1",column="1",row:column=c("0","0")))
print(con,X=TRUE)
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
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