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st: Omnibus effects following xtmelogit with margins


From   "Ploutz-Snyder, Robert (JSC-SK)[USRA]" <robert.ploutz-snyder-1@nasa.gov>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Omnibus effects following xtmelogit with margins
Date   Fri, 24 Sep 2010 10:26:58 -0500

Happy Friday Statalisters.  

I'm hoping someone can verify that -testparm- works appropriately to test the highest-order ominibus interaction effects following -xtmelogit- (as it works with xtmixed, for example). I also ran into a snag using -margins- as I usually do after an -xtmixed- model to obtain omnibus main effects for an ordinal factor variable.

 My model includes an indicator for time (three times) and an indicator for group (two groups) plus two continuous covariates. 

xtmelogit y i.time##i.group cov1 cov2 >0 ||subj: ||time:, nolog

...output omitted...  


My test of the omnibus interaction effect  (as I would run following xtmixed) revealed:

. testparm time#group2

 ( 1)  [eq1]2.time#10000.group2 = 0
 ( 2)  [eq1]3.time#10000.group2 = 0

           chi2(  2) =    6.46
         Prob > chi2 =    0.0396

But my use of -margins- to get the omnibus main effect for time (as I would with xtmixed) failed:

. margins time, asbalanced atmeans
default prediction is a function of possibly stochastic quantities other than e(b)
r(498);


Any assistance on this error?  Rob

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