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Re: st: xtreg or xtmixed?


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
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

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


http://www.maartenbuis.nl
--------------------------
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