[R] why NA coefficients
David Winsemius
dwinsemius at comcast.net
Tue Nov 8 02:13:16 CET 2011
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
More information about the R-help
mailing list