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st: Pweight


From   Gao Liu <gliu@rdmc.org>
To   statalist@hsphsun2.harvard.edu
Subject   st: Pweight
Date   Wed, 17 Sep 2008 12:53:24 -0400

Dear Statalist:

We are estimating the impact of an intervention on the practice of
health providers.  One outcome measure is the ratio of metformin
prescribed to diabetic patients. This ratio is defined as the ratio
between the number of diabetic patients prescribed with metformin and
the total number of diabetic patients treated by the provider. We try to
examine whether metformin ratios are different for treatment group and
unexposed group before and after the intervention date. Xtgee is used to
estimate the impact of the intervention.
 
But there is a problem: the number of diabetic patients varies a lot
from one provider to another.  Some had only one or two diabetic
patients in a quarter, while others had more than 50. If a provider has
only 2 diabetic patients, one of whom was prescribed with Metformin,
then one patient's drop from being prescribed with metformin would lead
to a change of 50% in the metformin ratio.  In contrast, a health
provider with 100 diabetic patients will have very stable metformin
ratio. Thus, it is inappropriate to treat all providers the same in the
regression.

Should we use the pweight option in the regression? Or is there any
other better approach that works for this study? Thank you



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