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# Re: st: xtreg or xtmixed?

 From Maarten Buis To statalist@hsphsun2.harvard.edu Subject Re: st: xtreg or xtmixed? Date Mon, 19 Dec 2011 11:02:30 +0100

```-xtreg- and -xtmixed- with one level of nesting should give you
(almost) exactly the same results. -xtreg- is however optimized code
for the problem with one level of nesting, so you should use that.

However, I can see a major problem in your analysis: You state that
your independent variable. In terms of Stata that means that you
should put neuropathy first.

Whether your explanatory variables are normally distributed is
irrelevant. The possible non-linearity of their effect is a real issue
that you need to address. Whether your dependent variable looks like a
normal bell shaped curve is actually also deceiving. I gave a talk on
that at the last Nordic Stata Users' Group meeting:
<http://www.maartenbuis.nl/presentations/stockholm11.pdf>.

Hope this helps,
Maarten

On Mon, Dec 19, 2011 at 10:44 AM, Johan van der Watt wrote:
> Hi
> I need to do longitudinal statistics on a panel data set of 150
> individuals over 4 different time points. My dependent variable is the
> development of neuropathy (everyone is neuropathy free at baseline
> assessment), and a have numerous continuous independent
> variables/possible risk factors for development of neuropathy, such as
> CD4 count. A few of the continuous variables are not normally
> distributed, and I am able to normalize most of them by either logging
> or square rooting them. Further, looking at these variables over time,
> some of them also do not follow a linear pattern, i.e they peak at
> visit 2 or 3 then go down at visit 4.
> I have used the -xtreg- command with -mle-, using a continuous
> variable, such as CD4 count, Neuropathy, and Visit:
>
> xtreg cd4_  neuropathy visit, i(patientno2) mle
>
> but after reading online articles and bulletin boards,  I have also
> tried the mixed effects model:
>
> xtmixed cd4_  neuropathy visit ||  patientno2:,mle
>
> As I am a stats amateur, is this what I am doing correct? Any other options?
>
> Thanks
>
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--
--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

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

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