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st: random quadratic effect without random linear effect?

From   Jordan Silberman <>
Subject   st: random quadratic effect without random linear effect?
Date   Wed, 19 Jun 2013 18:07:16 -0400

Hi Stata Folks,

For a model run in xtmixed, we all know that if you include a fixed
quadratic effect of predictor A, then you typically should include the
lower order fixed linear effect of A as well.

Does an analogous rule apply to random effects? I think that it may
make theoretical sense to include only a random quadratic effect,
without the corresponding random linear effect. Can anyone tell me
whether or not this would be "kosher?"

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