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Re: st: ANOVA repeated measures


From   "JVerkuilen (Gmail)" <jvverkuilen@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: ANOVA repeated measures
Date   Tue, 6 Nov 2012 09:04:48 -0500

Hmmm, well you have a few choices as to how to analyze these data. One
is simply to convert to difference scores (post - pre) and do a
two-sample t-test comparing the flush and non-flush group. This uses
the subjects as their own controls.

There is an equivalent linear mixed model (depending on how you
estimate the model it will be exactly equivalent, or just close). Wit
the data laid out long, use -xtmixed- you'd use a subject indicator
and flush as a fixed effects predictor.

You could also decide to use an ANCOVA approach. Reshape wide and use
the pre as a regressor along with the intervention.

It's not 100% clear which is the right thing to do. A lot depends on
how correlated pre and post are likely to be for the controls.



On Tue, Nov 6, 2012 at 8:55 AM, Rune Nielsen <nielsenrune@me.com> wrote:
> Dear statalist members,
>
> We have done a simple pilot study where we measure the number of bacteria on
> the tip of a bronchoscope two times on the same 20 subjects. Half of these
> subjects have received an intervention to reduce the number of bacteria. So
> in a long dataset with 40 observations  I have the following variables
> Idnr - subject ID
> meas - binary variable indicating first (=1) or second (=2) measurement
> flush - binary variable whether the subject have received (=1) or not (=0)
> the intervention
> bal - measurement of bacterial load
>
> What I would like to do, is to test whether the difference between
> measurement 1 and measurement 2 is  depending on whether they have received
> the intervention. I've tried various ANOVA syntaxes, but my limited
> knowledge won't quite get me there.
>
> Probably this reveals my incompetence, but nevertheless I hope for an answer
> that is understandable for a non-statisician.
>
> Best wishes,
>
> Rune Nielsen
>
> ---
> Rune Nielsen, MD, PhD, postdoctoral fellow
> Institute of Medicine
> Department of Thoracic Medicine
> Haukeland University Hospital
> N-5021 Bergen
> Norway
>



-- 
JVVerkuilen, PhD
jvverkuilen@gmail.com

"Thus the typical citizen drops down to a lower level of mental
performance as soon as he enters the political field. He argues and
analyzes in a way which he would readily recognize as infantile within
the sphere of his real interests. He becomes a primitive again. His
thinking becomes associative and affective."  ---Joseph A. Schumpeter,
Capitalism, Socialism and Democracy, 1950, p. 262.
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