[R] Nolinear mixed-effects models (nlme)

[Douglas Bates]

On 6/30/05, Alex Bach <alex.bach at irta.es> wrote:
> Hello,
> 
> I am trying to fit a nonlinear model of the form of:
> 
> A*x^b*exp(-c*x)
> 
> This represents a lactation curve. I have a bunch of cows, so I want
> COW to be a random effect.

You need to decide which of the model parameters (i.e. A, B and C)
should have a random effect grouped by COW and to specify this in your
call to nlme.

> 
> I have been trying the following code with very littel success:
> 
>  > fm1 <- nlme(yield ~ A*(DIM^B)*(exp(-C*DIM)),
> +             data = group,
> +             fixed = A + B + C ~ 1,
> +             start = c(A = 20, B = 0.3, C = 0.03))
> 
> Does anyone know how to add the random effect of the cow? I have used
> the command groupedData to have Cow as subject (i.e., yield~DIM |
> cow). Is this a valid and sufficient approach? I have the feeling it
> is not sufficient.
> 
> Also, does anyone know whether the formulation of the fixed effects
> is correct?.
> 
> Thank you very much,
> 
> Alex
> 
> 
> 
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