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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 10:14:37 -0500

regress mydepvarpost mydepvarpre treatment



On Tue, Nov 6, 2012 at 9:28 AM, Rune Nielsen <nielsenrune@me.com> wrote:
> Thank you for your kind answer.
> The linear mixed model is probably to difficult for me at the moment since
> I'm just trying to summarize some preliminary analyses and don't have the
> opportunity right now to do extensive reading.
>
> I've tried analyzing on the difference scores, but got the impression of
> somehow loosing power. When I reshaped the dataset (to wide), stratified by
> flush and did t-tests on the difference I found significant results (bysort
> flush: ttest bal1=bal2). But when I did as you recommended they where not
> even close to significant (ttest baldiff by(flush)).
>
> Would you be able to provide an example of an ancova syntax?
>
> Again, thank you!
>
> Best wishes from Rune
>
> ---
> Rune Nielsen, MD, PhD, postdoctoral fellow
> Institute of Medicine
> Department of Thoracic Medicine
> Haukeland University Hospital
> N-5021 Bergen
> Norway
>
> 6. nov. 2012 kl. 15:04 skrev "JVerkuilen (Gmail)" <jvverkuilen@gmail.com>:
>
> 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.
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
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> *   http://www.ats.ucla.edu/stat/stata/
>
>



-- 
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.
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/


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