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Re: st: RE: psmatch2 and p-value


From   "Dimitriy V. Masterov" <dvmaster@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: psmatch2 and p-value
Date   Thu, 2 Dec 2010 09:47:03 -0500

Colette,

My understanding is that there is no deterministic variable selection
procedure that will tell you which variables to choose from those
available to do matching. However, more is better is not quite right.

You should try to include all variables that affect both participation
and the outcome variable. This information comes from theory,
institutional knowledge, and previous research. Including instruments
– variables that affect participation and not outcomes – is not a good
idea. They do not help with selection bias and may worsen the support
problem. I am not sure if this is a consensus view. Perhaps others can
chime in.

DVM

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