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Re: Re: st: How to graph means with correct SE after repeated ANOVA?


From   "Airey, David C" <david.airey@Vanderbilt.Edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: Re: st: How to graph means with correct SE after repeated ANOVA?
Date   Fri, 26 Mar 2010 12:33:43 -0500

.

> I looked up http://www.ats.ucla.edu/stat/stata/faq/margins_nested1.htm
> 
> where you see a similar situation for the SE. 
> 
> WHY ARE THE SE exactly the SAME? In particular, the consequence is, that
> the confidence intervals in one group have the same range (i.e. the same
> absolute difference). Is this a mistake or correct? If this is correct
> then graphing confidence intervals in this case would not make a lot of
> sense...!
> 
> Thank you so much for any advice!
> 
> Best Linn

In that example, it's also possible to use

margins, within(trt)

You use within() with nested designs.

It does seem a little confusing, since within() is required to estimate predicted marginal means in some models, but not others:

/* one between, one within-subject factor design */
use http://www.ats.ucla.edu/stat/data/repeated_anova, clear

anova dv trt / sid|trt time trt#time, repeated(time)

margins trt           // doesn't work
margins, within(trt)  // works

/* one between, two-within-subject factor design, Example 17 of the Stata 11 manual for -anova- */

use http://www.stata-press.com/data/r11/t713, clear

anova score noise / subject|noise period noise#period ///
	/ period#subject|noise dial noise#dial / dial#subject|noise ///
	period#dial noise#period#dial, repeated(period dial)

margins noise           // works
margins, within(noise)  // works

Why is that???

Note, with regard to your question, the standard errors for the means within subject factors are the same, as in your model without the between subject factor. So we need to know the relationship between the standard errors for those marginal means (which are the same across within subject factors), and the mean squared errors used in the ANOVAs for testing the within subject terms (which differ).

So we have more questions.

-Dave


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