why NA coefficients
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? Thanks John -------------- next part -------------- An embedded and charset-unspecified text was scrubbed... Name: test.txt URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20111107/430e48a8/attachment.txt>