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st: Modeling animal growth - nonlinear mixed model


From   "Burt Staniar" <wstaniar@das.psu.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Modeling animal growth - nonlinear mixed model
Date   Mon, 22 Sep 2008 16:27:37 -0400

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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