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re: st: margins after split-plot anova


From   "Airey, David C" <david.airey@vanderbilt.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   re: st: margins after split-plot anova
Date   Sat, 23 Jun 2012 09:19:46 -0500

.

One source for adding a covariate to a split plot design is Chapter 11 in Variations on Split Plot and Split Block Experimental Designs (Wiley 2007).

Cheers,

-Dave

> Thank you. 
> In the following example the option at() does not work (not estimable): 
> 
> anova depvar c.IQ group / id|group label label#group label#c.IQ , repeated (label) 
> margins, within(group) at( IQ=(1))
> 
> There is something about having a continuous covariate in the anova model that I’m missing or that does not work in Stata. 
> 
> Let’s look at the following examples: 
> - no continuous covariate: 
> 1) anova depvar group / id|group label label#group, repeated (label) 
> margins, within(group)
> 
> 2) regress depvar group label label#group 
> margins group
> 
> - with continuous covariate: 
> 3) anova depvar c.IQ group / id|group label label#group label#c.IQ , repeated (label) 
> margins, within(group)
> 
> 4)regress depvar c.IQ group label label#group label#c.IQ 
> margins group
> 
> 
> The output of margins in 1), 2), AND 3) are equal. The output of 4) is (correctly) different from 1) and 2), because 4) includes a continuous covariate that is not present in 1) and 2). 
> And the problem (again, perhaps I’m missing something simple, but I don’t know what) is that the outcome of margins in 3) is equal to the outcome of margins in 1) and 2). 
> 
> Thank you again for any help anybody can provide. 
> Luca


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