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# Re: Re: st: Propensity score matching: confidence intervals

 From "Ariel Linden. DrPH" To Subject Re: Re: st: Propensity score matching: confidence intervals Date Wed, 27 Jul 2011 09:33:49 -0700

First off, you have not been clear with us on what you are measuring. I
suggested using regression manually (outside of PSMATCH2) and add in either
robust or clustered standard errors. Your response was that you are using
logistic regression.

We cannot be expected to provide meaningful help if you do not provide us
sufficient information.

Are you using binary outcome variables or continuous? PSMATCH2 does not
differentiate and will treat your outcomes using OLS regression.

How many matched controls are you using for each treated case? If you have a
N to 1 match, you would need to account for this by weighting the
observations within each matched group).

My original suggestion stands, and that is to perform the outcome
measurement manually after PSMATCH2 identifies the matches for you (and the
weights, assuming you have an N to 1 matching strategy). This will allow you
to run your regressions  with the appropriate standard errors (accounting
for the dependent nature of the data) .

Date: Wed, 27 Jul 2011 00:41:21 +0200
From: Nyasha Tirivayi <ntirivayi@gmail.com>
Subject: Re: st: Propensity score matching: confidence intervals

Thank you for the responses. I am bit confused. in the psmatch2 code,
there is no option for vce or robust. psmatch2 appears to only allow
bootstrapping, even though the software authors say its unclear if its
valid. My core problem is obtaining confidence intervals.
Bootstrapping gives me confidence intervals based on the normal
distribution. I am interested in reporting the t-statistic. How can I
obtain confidence intervals that correspond to the t-statistic?

Secondly my t-statistic indicates strongly significant effect. After
bootstrapping, the p-value (normal distribution) is marginally
significant. Which p-value do I base my interpretation, the
t-statistic or the one after bootstrapping?