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st: gllamm and u-shaped cumulative logits over time

From   Thomas Herold <>
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.

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