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re: st: Propensity Score Matching (PSM) - matching problem

From   "Ariel Linden. DrPH" <>
To   <>
Subject   re: st: Propensity Score Matching (PSM) - matching problem
Date   Thu, 6 Sep 2012 05:41:33 -0400

The only reason I can think of that you'd have this issue arise is if your
treatment group is not consistently coded as 1 and your non-treatment group
is not consistently coded as 0.

There are basically two ways to handle "matched" data over time. The first
would entail matching (or weighting) on pre-intervention data and assume
that the treatment status never changes over time. In this case, you'd
control for pre-intervention covariates by matching (or weighting) and not
do any further adjustments after the intervention starts.

The second would assume that the treatment status could possibly change over
time (as well as allowing for time-varying covariates). In health-care this
could happen if a treatment (let's say a drug) is given at some point, and
then not given, and then given again. This is obviously a much more
complicated model to deal with than the first scenario (See Hernán et al*
for a good description of such an approach)...

I am not sure this is the issue with your data, but it seems to be a
reasonable assumption based on the limited information you provided....

I hope this helps


* Hernán, M. A., Brumback, B. & Robins, J. M. (2002) Estimating the causal
effect of zidovudine on CD4 count with a marginal structural model for
repeated measures. Statistics in Medicine, 21, 1689?1709. 

Date: Thu, 6 Sep 2012 02:40:27 +0300
From: umut senalp <>
Subject: st: Propensity Score Matching (PSM) - matching problem

Dear Statalisters,
I am currently working with a panel containing around 9.000 firms and
trying to evaluate the exports effects on firm-level productivity, and
I use Matching approach (Propensity Score Matching) to get the
treatment/export effects (on treated - ATT).
I use -psmatch2.ado- (writen by Edwin Leuven and Barbara Sianesi) 
I already matched the exporters with non exporters. However, I am
having some problems with the application of the method. The problem 
is PSM procedure allows the PSM algorithm to match
a treated firm with a treatment-group firm (and sometime itself) 
after the treatment is over, by considering
treated firmsafter treatment as if they were control firms.
What I would like to ask is if is there any solution about the problem.
I would be glad if you could provide me a hint on this issue.
Kind regards 	

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