On Nov 7, 2011, at 7:33 PM, array chip wrote:
Hi, I am trying to run ANOVA with an interaction term on 2 factors
(treat has 7 levels, group has 2 levels). I found the coefficient
for the last interaction term is always 0, see attached dataset and
the code below:
test<-read.table("test.txt",sep='\t',header=T,row.names=NULL)
lm(y~factor(treat)*factor(group),test)
Call:
lm(formula = y ~ factor(treat) * factor(group), data = test)
Coefficients:
(Intercept)
factor(treat)2 factor(treat)3
0.429244
0.499982 0.352971
factor(treat)4
factor(treat)5 factor(treat)6
-0.204752
0.142042 0.044155
factor(treat)7 factor(group)2
factor(treat)2:factor(group)2
-0.007775
-0.337907 -0.208734
factor(treat)3:factor(group)2 factor(treat)4:factor(group)2
factor(treat)5:factor(group)2
-0.195138
0.800029 0.227514
factor(treat)6:factor(group)2 factor(treat)7:factor(group)2
0.331548 NA
I guess this is due to model matrix being singular or collinearity
among the matrix columns? But I can't figure out how the matrix is
singular in this case? Can someone show me why this is the case?
Because you have no cases in one of the crossed categories.
--
David Winsemius, MD
West Hartford, CT
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