R help - Oct 2011 - Thank you

Dear r-community,

Today I have completed my PhD.  I would like to take this opportunity to thank
the r-community for helping me with the r-coding.  I use r to do data
manipulation during my PhD and I benefit a lot through the discussion in the
r-forum.

I will continue using R and help the others too.

Thank you so much.

Regards,

Roslina
UniSA.

	[[alternative HTML version deleted]]

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)

Confidentiality Statement:
This email message, including any attachments, is for th...{{dropped:6}}

... and while I am at it, as this is the U.S. Thanksgiving...

My sincere thanks to the many R developers and documenters who
contribute large amounts of their personal time and effort to
developing, improving, and enhancing the accessibility of R for data
analysis and science. I believe it is fair to say that R has had as
much or more impact than Gosset's Student's T, and I fear that
academics who do much of this work do not receive the professional
recognition they deserve. I continue to be amazed and humbled by their
high quality and consummate professionalismism -- I could not live
without R.

Kind regards and best wishes to all,

-- Bert

-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm