[R] Nolinear mixed-effects models (nlme)

From: Alex Bach <alex.bach_at_irta.es>
Date: Fri 01 Jul 2005 - 09:09:28 EST


I am trying to fit a nonlinear model of the form of:


This represents a lactation curve. I have a bunch of cows, so I want COW to be a random effect.

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,


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