[R] Nested error structure in nonlinear model

[Spencer Graves]

	  Have you read Pinheiro and Bates (2000) Mixed-Effects Models in S and 
S-Plus (Springer).  If no, I believe that book should help you.  If you 
have access to the library at the University of Waikato, as suggested by 
your email address, then I suggest you try the library there.  The 
electronic catalog just told me it had a paper copy listed as 
"available" as well as an electronic copy.

	  If you have read Pinheiro and Bates, then please prepare another post 
more consistent with the posting guide 
"www.R-project.org/posting-guide.html".  Pinheiro and Bates and the 
documentation for "nlme" contain several examples that should help you 
describe succintly where you are encountering difficulties.  If someone 
like me can copy a few lines of code from an email, paste it into R and 
reproduce what you are seeing in a few seconds, you are much more likely 
to get quality help quickly than if I have to guess -- and at least one 
of the leading contributors to this list has announced a personal policy 
of rarely replying to posts whose style is not consistent with that guide.

	  hope this helps,
	  spencer graves

Murray Jorgensen wrote:

> I am trying to fit a nonlinear regression model to data. There are 
> several predictor variables and 8 parameters. I will write the model as
> 
> Y ~ Yhat(theta1,...,theta8)
> 
> OK, I can do this using nls() - but "only just" as there are not as many 
> observations as might be desired.
> 
> Now the problem is that we have a factor "Site" and I want to include a 
> corresponding error component. I tried something like (excuse the 
> doctored R output):
> 
>  > mod0.nlme <-  nlme(Y ~ Yhat(theta1,...,theta8) + site.err ,
> +                    start = c(mod0.st,0),
> +                    groups = ~ Site,
> +                   fixed = theta1 + ... + theta8 ~ 1,
> +                   random = site.err ~ 1)
> Error in nlme.formula(Y ~ Yhat(theta1,...,theta8  :
>          starting values for the fixed component are not the correct length
> 
> and then
> 
>  > mod0.nlme <-  nlme(Y ~ Yhat(theta1,...,theta8) + site.err ,
> +                    start = mod0.st,
> +                    groups = ~ Site,
> +                   fixed = theta1 + ... + theta8 ~ 1,
> +                   random = site.err ~ 1)
> Error: Singularity in backsolve at level 0, block 1
> 
> The straightforward way would be to go
> 
>  > mod0.nlme <-  nlme(Y ~ Yhat(theta1,...,theta8) ,
> +                    start = mod0.st,
> +                    groups = ~ Site,
> +                   fixed = theta1 + ... + theta8 ~ 1)
> 
> making all 8 parameters random, but there is little hope of getting that 
> to work. (I did try it and it does not crash straight away but settles 
> down to run forever.)
> 
> Any comments welcome. I regret that I cannot go into much detail about 
> the actual problem.
>