Is X a numeric variable or a factor? If it's numeric try....
gam.lme<- gamm(Y~ s(z, by=X), random=list(groups=pdDiag(~1+X)) )
... since otherwise the separate X term is confounded with s(z, by=X).
(gam detects such confounding and copes with it, but gamm can't).
Simon
On 17/03/11 17:47, Irene Mantzouni wrote:
Hi all,
I would like to fit a gamm model of the form:
Y~X+X*f(z)
Where f is the smooth function and
With random effects on X and on the intercept.
So, I try to write it like this:
gam.lme<- gamm(Y~ s(z, by=X) +X, random=list(groups=pdDiag(~1+X)) )
but I get the error message :
Error in MEestimate(lmeSt, grps) :
Singularity in backsolve at level 0, block 1
When I simply fit a gam model using the formula above, then it works ok.
Is it possible to fit such a model with gamm?
Thanks a lot!
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Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
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