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Re: st: Modeling animal growth - nonlinear mixed model


From   David Airey <david.airey@Vanderbilt.Edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Modeling animal growth - nonlinear mixed model
Date   Mon, 22 Sep 2008 15:39:20 -0500

.

I don't think there are any nonlinear mixed models in Stata (there is a vce option for clusters in nl, but that's it). NLME in R is an option if you have lost access to SAS, but sticking with SAS for this problem seems best since you've done it before. Both xtmixed and gllamm are for linear models or latent linear models (no exponentiated coefficients). I'm sure someone will correct me if I'm wrong.

-Dave

On Sep 22, 2008, at 3:27 PM, Burt Staniar wrote:


I have a few questions in regard to my attempts to model animal growth
using Stata. I am using some basic sigmoid equations to model equine
growth. I have previously done this in SAS, but have moved to Stata for
most of my other work. I would like to be able to do this work in
Stata. I have a longitudinal data set in a long format, with columns
for age, day of year, weight and height. I have successfully used the
following model but would like to go a few steps further:

nl (weight ={A=500}*(1-{b=0.9}*exp(-{k=0.003}*age))^{M=0.5}),
cluster(animal_id)

1) How can I produce prediction or tolerance intervals for the model?
2) How can I produce percentile curves?
3) In SAS, I used nlmixed to fit both fixed and random effects. The
best results were placing random effects associated with animal_id on
the A and k parameters. Is this possible using Stata? I have explored
xtmixed and gllamm, but without results.

Thank you for any help that may be offered.

W. Burton Staniar
Assistant Professor of Equine Nutrition
The Pennsylvania State University
324 W. L. Henning Building
University Park, PA 16802
814-865-0698
wstaniar@psu.edu


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