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Re: st: Imbalance in control versus treated group, and weights


From   "Martin Weiss" <martin.weiss1@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Imbalance in control versus treated group, and weights
Date   Thu, 9 Oct 2008 22:59:18 +0200

http://www.stata-journal.com/article.html?article=st0136

HTH
Martin
_______________________
----- Original Message ----- From: <Alexander.Severinsen@telenor.com>
To: <statalist@hsphsun2.harvard.edu>
Sent: Thursday, October 09, 2008 10:56 PM
Subject: SV: st: Imbalance in control versus treated group, and weights


I have another question. I followed the advice and looked into propensity score reweighting (PSR) and regression discontinuity (RD). Google pointed me to Austins presentation about this topis, http://www.stata.com/meeting/6nasug/causal.pdf

I have read through the presentation, but I do not understand all the assumptions that underpins RD. My problem pass the first assumption that my treatment is not randomly assigned, though it started out as a randomized controlled trial, just that not all those supposed to have a treatment got one. Further, the assignment variable is based on a observable variable. Or well, it was not supposed to be an assignment variable, but it turned out to be, and consequently contaminated the treated versus the control group.

However I am uncertain what the second assignment is telling me, quoting Austins presentation

"The crucial second assumption is that there is a discontinuity at some cutoff value of the assignment variable in the level of treatment."

My assignment variable do produce a jump in the level of treatment, but I am unsure whether this actually means that I pass assumption 2?

I also downloaded the RD package from SSC (findit regression discontinuity). However, I am still unclear how I can relate the provided example to my own problem. I am having trouble locating other examples, and any tip would be greatly appreciated.

Best wishes,
Alexander Severinsen


-----Opprinnelig melding-----
Fra: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] På vegne av Alexander.Severinsen@telenor.com
Sendt: 8. oktober 2008 19:11
Til: statalist@hsphsun2.harvard.edu
Emne: SV: st: Imbalance in control versus treated group, and weights

Thank you for the advice. Very helpful!

In this spesific case z is a dummy, and if z=1 then this will increase the likelihood of observing x=1. And yes, I do observe outcomes for the group that was supposed to be treated, but were not.

Best wishes,
Alexander

-----Opprinnelig melding-----
Fra: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] På vegne av Austin Nichols
Sendt: 8. oktober 2008 18:39
Til: statalist@hsphsun2.harvard.edu
Emne: Re: st: Imbalance in control versus treated group, and weights

It is possible that some kind of propensity score reweighting or regression discontinuity design would be appropriate here, but without much more information, it is hard to offer any specific advice. How does z affect x in the group supposed to have x=1? Do you observe outcomes for the group supposed to have x=1 but having x=0? Etc.

Running a probit with the assumption E(y)=F(b0+b1*x+b2*z) seems unlikely to recover a good estimate of the effect of x on y unless that assumption is actually true!

On Wed, Oct 8, 2008 at 12:23 PM, <Alexander.Severinsen@telenor.com> wrote:
Dear Statalisters,

I have the following problem. I have given a sample of 10000 people as targets for receiving an offer, and I have a control group equal to 5000 people. I know that the potentially treated and the controlgroup is representative. However, without my knowledge only 8000 of the 10000 targets were treated, and a specific criteria was used to pick those 8000 from the 10000.

This has created an imbalance between my controlgroup and those treated, and this imbalance is identified and only concerns one variable. I want to investigate whether the offer given could reduce the defection rate of customers, but the variable that created this imbalance is known to hugely impact the defection rate. To reduce this problem I would like to use weights in Stata, but I am unsure on how to approach this? Any tips would be greatly appreciated.

Also, say that I did not correct for this, and did the following probit model with the following variables, y=defected/not defected, x=treated/control, z=factor that created imbalance:
       y=b0+b1*x+b2*z
would it be appropriate to say that it was possible to control for the imbalance by including it as a independent variable in this fashion?

Best wishes,
Alexander Severinsen
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