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st: gllamm and u-shaped cumulative logits over time
From
Thomas Herold <[email protected]>
To
[email protected]
Subject
st: gllamm and u-shaped cumulative logits over time
Date
Tue, 19 Mar 2013 20:17:34 +0000
Dear statalist users,
I am working on a longitudinal dataset dealing with depression, measured
as ordinal variable (1: none, 2: mild, 3:moderate, 4: severe). There are
5 measurement points (t0-t4) and two different groups (T0=conventional
treatment GP, T1=new treatment). I want to find out whether the
treatment has an influence on the development of the depression score. I
would normally use the -gllamm- command with the model (without a random
slope) looking something like this:
gllamm depr week treatment interact, i(id) link(ologit) adapt eform
, where week is the time tx-t0 and interact=week*treatment.
But here is my problem: The development over time (and over time by
group) is not linear but clearly (!) u-shaped. There is a short-term
effect in both groups, i.e. the depression scores decline considerably
from t0 to t1. However, in the long run (t1-t4), the scores in both
groups increase again and often even exceed the initial value at t0. For
obvious reasons, the above model fits the data very poorly.
Any help would be much appreciated.
Best,
Thomas
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