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st: Predicting multiple slopes with xtmixed


From   Robie Sterling <rorsterling@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Predicting multiple slopes with xtmixed
Date   Mon, 22 Jun 2009 09:52:59 -0700

Hi statalisters,

I am attempting to estimate the individual slopes of two time periods
in nonlinear mixed models of unbalanced panel data of weight for
length SD values (wlz) over time, but am unclear as to the best method
using xtmixed. Graphing the data shows growth from 0-6 mos has a
different trajectory than growth after 6 mos.

I would like to estimate the slope from age=0 to age=6, and then have
another slope estimate for the growth thereafter for each
participant(id). Can this be done with one xtmixed model? From the
gutierrez pdf and the statalist post from Jose Maria Pacheco de Souza,
I could not find a similar example.

my data set looks like this:

id, age, age6+, wlz
1     1      0       0.5
1     2      0       0.45
1     3      0       0.3
1     5      0       0.05
1     6      0      -0.02
1     7      1      -0.1
1     9      3      -0.21
2     5      0       0.70
2     6      0       0.72
2     7      1       0.78
2     8      2       0.80
2    10     4       0.84
...
200  12    6       -0.50

thanks so much

Robie Sterling
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