Hi Chia,
as far as I know there is no implementation of fixed covariates in nlmer()
yet. I have to admit that I have given up op that a few month ago. I went
on to fit my model using (orthogonal) polynomials of my covariate using the
lmer(). It's not pretty and has some convergence problems due to high
correlations of the random effects, but it's still better than a linear fit.

Some while ago, Ben Bolker provided some insights about this problem here:
http://rpubs.com/bbolker/3423. As far as I can tell, he went on to code the
covariates in the model function. He succeeded in getting the
nlmer()-funtion to converge, but as he says "We *can* get an answer (*but
not a sensible answer*) with the stable (lme4.0) version."

I took the liberty to CC the r-sig mailing list again, perhaps somebody out
there knows more about it...

Best regards
Marko



2013/4/23 <kohleth.chia@dpi.vic.gov.au>

> Hi Marko,
>
> Have you worked out how to include fixed effects in nlmer? it seems to me
> that it is still not implemented.
>
> --------------
> *Kohleth Chia*
> Research Scientist - Biometrics
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>
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