[R] fitting gamm with interaction term

[Simon Wood]

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,
>
>
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> I would like to fit a gamm model of the form:
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>
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> Y~X+X*f(z)
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> Where f is the smooth function and
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> With random effects on X and on the intercept.
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> So, I try to write it like this:
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> gam.lme<- gamm(Y~ s(z, by=X) +X,  random=list(groups=pdDiag(~1+X))  )
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>
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> but I get the error message :
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> Error in MEestimate(lmeSt, grps) :
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>    Singularity in backsolve at level 0, block 1
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>
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> When I simply fit a gam model using the formula above, then it works ok.
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>
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> Is it possible to fit such a model with gamm?
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> Thanks a lot!
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-- 
Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
+44 (0)1225 386603               http://people.bath.ac.uk/sw283