st: problem with bsample

[Woolton Lee <finished07@gmail.com>]

I am having an odd problem with STATA's bsample command.  I have a
discharge dataset where each row of the dataset represents a patient
zip code hospital zip code combination.  In other words patients from
one zip code that were admitted to a hospital in the hospital zip
code.  Some patient zip codes fall into a treatment group (tx = 1) and
some which fall in a comparison group (tx = 0).  We are using the
following command (essentially)

bsample , strata(tx) cluster(patzip)

this stratifies the sampling by treatment and control group and
clusters the draws by patient zip code.  However, for one of our CA
markets we are finding that the bsample obtains a draw that contains
no observations from the treatment group - (which given the command
above doesn't make any sense).  This particular market has roughly 95%
of its observations in the comparison group and 5% of observations in
the treatment group.  But given that bsample is supposed to be drawing
its samples stratified by treatment/comparison group this still should
not happen.  Has anyone seen this before?  If so, do you have any
ideas how to handle it?

Thanks for your help,

Woolton
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