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Re: st: xtreg or xtmixed?

From   Maarten Buis <>
Subject   Re: st: xtreg or xtmixed?
Date   Mon, 19 Dec 2011 13:18:13 +0100

On Mon, Dec 19, 2011 at 12:39 PM, Johan van der Watt wrote:
> The question that I would like to answer, is whether the course of CD4
> count over time is different in the group of patients who have
> neuropathy vs the group who do not have neuropathy. Firstly, my
> dependent variable is binary, either yes or no for development.

If that is your question than neuropathy is _not_ your
dependent/explained/left-hand-side/y variable but your
independent/explanatory/right-hand-side/x variable. The dependent
variable is the thing you want to explain, in this case CD4 count. The
independent variable is what you use to do the explaining, in this
case neuropathy.

> Secondly, I am struggling with the fact that some of the continuous
> variables are non-linear when I plot them over time, in other words,
> as an example, I find inflammatory markers going up from baseline
> between the 2nd and 3 visit, and then going back to baseline level at
> the last visit. This pattern is the same in both groups (neuropathy
> and neuropathy free), however I suspect the magnitude might be
> different.

That sounds like you want to add dummies for three of your time
periods (treat baseline as the reference) and add interactions between
neuropathy and the time dummies.

> Thank you for the slides, I am just not sure how to deal
> with this in longitudinal data.

The point you need to take from that is not expect that the
distributions of your dependent variables are normally distributed.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
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