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RE: st: repeated measures ANOVA to MANOVA - revisit [on behalf of Rob Ploutz-Snyder]


From   "Feiveson, Alan H. (JSC-SK311)" <alan.h.feiveson@nasa.gov>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: repeated measures ANOVA to MANOVA - revisit [on behalf of Rob Ploutz-Snyder]
Date   Fri, 3 Feb 2012 09:27:39 -0600

Ricardo..
I have to be honest... I've abandoned Stata's implementation(s) of repeated measures ANOVA entirely, as I am rarely able to get it to replicate what I know to be accurate with other software (sorry Stata).  Likely this is due to my confusion about how Stata wants us to program something as simple as a repeated measures ANOVA, and my personal impatience with that.  It isn't as straightforward as it should be.  

You will find help on Phil Ender's pages at USLA (http://www.ats.ucla.edu/stat/stata/ado/analysis/default.htm) where there are several nicely written ado's that'll help you understand how to use Stata to do typical ANOVA routines.  

Having said that, mixed-effects modeling such as implemented in Stata's -xtmixed- has distinct advantages over RM ANOVA, making the latter a tool that I find myself using less and less.  If you have a completely balanced factorial design and you meet ALL of the assumptions of traditional RM ANOVA, then RM ANOVA offers the benefit of a formal F-test that doesn’t rely on maximum likelihood estimation.  It's also somewhat easier to write-up & defend for manuscripts, as reviewers (who usually aren't statistician) seem to understand "ANOVA" better than "MIXED-EFFECTS MODELING," even though that shouldn't be the case.

But if you have imbalance in cell sizes, occasional missing data here and there, and the potential for random effects in your data, the mixed-modeling (i.e. -xtmixed-) is likely going to be your better approach.  And... the coding is straightforward.  And with the new -contrasts- and -margins- and -marginsplot- post-estimation routines, -xtmixed- functionality is really quite nice for ANVOA-like factorial designs.  So given those advantages, I personally gravitate towards it from the get-go and do my best to educate manuscript reviewers along the way.


Best,
Rob



-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Ricardo Ovaldia
Sent: Friday, February 03, 2012 7:54 AM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: repeated measures ANOVA to MANOVA - revisit

I did not get a reply to my question, so here is the simplified version:

Using the 23 observations lited below I get very different p-values when using repeated measurements ANOVA and MANOVA. Why the descrepancy? Am I doing something wrong? Can I use -xtmixed- instead (how)?



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