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RE: st: Re: Repeated Measures ANOVA vs. Friedman test


From   Steve <cheesehead.in.tx@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   RE: st: Re: Repeated Measures ANOVA vs. Friedman test
Date   Tue, 22 May 2012 14:49:01 -0500

Dave, Joesph, Rob and Nick,

Thanks for all your replies.  As you might have gathered, my main
concern was whether the small sample size would be compatible with a
RM ANOVA.  Since this was panel data - and in the past I've used
-xtmixed- for it, I was leaning a bit towards that but was curious
about other analytic possibilities.

Also, Joesph, thanks for the detailed post with the info on the
diagnostic plots.  I had looked at those previously and there weren't
really any major outliers.  If anything the data seemed a bit
uniformly distributed.  However, I think the most important point you
made was the fact that since this is a pilot study, I should stop
fretting about the results and focus on the quality. :)  Incidentally,
using parametric vs non-parametric or even looking at differences
between values vs factoring in time all pretty much gave the same
result with p-values ranging from 0.50 to 0.25

Once again, thanks for all the help, guys.

- Steve




-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Airey,
David C
Sent: Tuesday, May 22, 2012 11:08 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: Re: Repeated Measures ANOVA vs. Friedman test

.

That's true; the contrast postestimation command will use a t after
regress/ANOVA but a z after xtmixed. On the other hand, the compound
symmetric covariance for regress/ANOVA may not be a good choice, and
the univariate corrections for departure from the sphericity
assumption are approximate. Also, with other than designed
experiments, missing time points are likely, and regress/ANOVA does
casewise deletion whereas xtmixed will retain cases missing one time
point.

> One more thing - if you can "get away with" the assumptions for ANOVA and everything is balanced, it should provide closer to nominal test levels than -xtmixed- because of the finite degrees of freedom for error in the former vs. asymptotic Z- statistics in the latter.
>
> Al Feiveson
>



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