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Re: st: Comparing change in rates - frustrating problem: questionable results


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Comparing change in rates - frustrating problem: questionable results
Date   Wed, 11 Feb 2004 05:58:23 -0800 (PST)

Thank you Joseph for your thoughtful analysis of this
problem. I had originally misunderstood the
interpretation of the ANCOVA like approach. I though
that the treatment effect was being tested by the
interaction term in the model. In fact, the treatment
effect is tested by the "treatment" term itself. So
just like in ANOVA, if the interaction term is not
significant then the main effect can be interpreted
directly. Is this correct?

One last thing, if the interaction term is not
significant, does it still need to be included in the
model?

Thank you very much for all your help.
Ricardo.


=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

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