Thanks David. The only category that has no cases is "treat 1-group 2":
> with(test,table(treat,group))
group
treat 1 2
1 8 0
2 1 5
3 5 5
4 7 3
5 7 4
6 3 3
7 8 2
But why the coefficient for "treat 7-group 2" is not estimable?
Thanks
John
________________________________
From: David Winsemius <dwinsem...@comcast.net>
Cc: "r-help@r-project.org" <r-help@r-project.org>
Sent: Monday, November 7, 2011 5:13 PM
Subject: Re: [R] why NA coefficients
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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