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st: using -mixed- with clustered data that includes probability weights


From   Stephen Henry <sghenry@ucdavis.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: using -mixed- with clustered data that includes probability weights
Date   Thu, 27 Feb 2014 16:55:58 -0800

Hi,

I want to know whether I can used -mixed- in Stata 13.1 to analyze
clustered data that include probability weights.

My data were collected to study patients during clinic visits.
Each patient is unique, and patients are clustered within doctors.
In addition, patients were sampled based on an illness severity score.
Patients with more severe symptoms were over-sampled.
Patient sampling was done independent of which doctor was seeing the
patient.

I have been analyzing data using the -reg- command and cluster option as
follows:

reg v1 v2 v3 [pweight=weight], cluster(doctor_id)

However, I'd like to use -mixed- instead to take advantage of the
additional postestimation commands.

Stata will run the following command:

 mixed v1 v2 v3 [pweight=weight] || doctor_id:

but warns me that "Sampling weights were specified only at the first level
in a multilevel model."
Are my results with the -mixed- command potentially biased?  If so, is
there an easy way to fix this?

Thanks in advance,

Stephen Henry
University of California Davis
Sacramento, California
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