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st: Standardized difference of means after PS matching


From   Adam Olszewski <adam.olszewski@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Standardized difference of means after PS matching
Date   Wed, 29 Aug 2012 00:15:15 -0400

Hello,
I noted something surprising today and I was wondering if any
stata-listers have some insight into this issue.
I have been using the (SSC derived user program) -psmatch2- for
propensity score matching and (also SSC-derived) -pbalchk- for
evaluation of balance after matching using standardized differences of
means (SDM).
I noted that the balance (sometimes quite dramatically!) improves if I
replace in the PS logistic model categorical variables with a 'plain'
non-factorized version (e.g. use "c.race" rather than "i.race" as a
variable). This of course makes no sense as a rational "real-world"
use of a variable, however the goal of PS analysis is to achieve the
balance and not to make a sensible predictive model.
I wonder however if this improvement could be an artifact of how the
SDM's are calculated by the -pbalchk- command (of course I calculate
categorized proportion differences using the -xi- syntax, ie.
"i.race"). I would not like to exploit a mathematical quirk to have
"better" (?) results, but on the other hand I can't find anything
particularly conceptually wrong with it.
Would any psmatch2/pbalchk users disagree?
Best regards,
Adam Olszewski
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