[R] Contrasts with an interaction. How does one specify the dummy variables for the interaction

[Daniel Malter]

Is there a specific reason why you insist on using the contrast library? If
not:

# 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)

#Create contrasts

row<-factor(row)
column<-factor(column)
contrasts(row)<-contr.treatment(levels(row))
contrasts(column)<-contr.treatment(levels(column))

# Works for Terps

fit.terp<-glm(counts ~ row + column + row*column,
family=poisson(link="log"))
summary(fit.terp)

HTH,

Daniel Malter
University of Maryland, College Park





John Sorkin wrote:
> 
> 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)
> 
> Confidentiality Statement:
> This email message, including any attachments, is for th...{{dropped:6}}
> 
> ______________________________________________
> R-help@ mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 


--
View this message in context: http://r.789695.n4.nabble.com/Contrasts-with-an-interaction-How-does-one-specify-the-dummy-variables-for-the-interaction-tp3948792p3949071.html
Sent from the R help mailing list archive at Nabble.com.