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st: RE: RE: psmatch2-identifying matched pairs


From   "Garth Rauscher" <garthr@uic.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: psmatch2-identifying matched pairs
Date   Mon, 5 Apr 2010 11:27:08 -0500

Hi Ariel, I'm sure that explains it. 

In an attempt to wrap up this thread, let me review my original question and
the solution that was provided.

Objective: To identify the matched pairs that are created by the psmatch2
command. 

The code below performs 1:1 nearest neighbor matching without replacement
and creates a new dataset named "paired" that contains the variable "pair"
that identifies which matched set each observation belongs to.

//--------------------------------------------------------------------------
psmatch2 xvar, pscore(pvar) outcome(yvar) caliper(.001) noreplace
neighbor(1)
gen pair = _id if _treated==0 
replace pair = _n1 if _treated==1
bysort pair: egen paircount = count(pair)
drop if paircount !=2
save paired, replace
//--------------------------------------------------------------------------

Having the variable "pair" enables me to use the dataset of matched pairs to
run matched analyses in other stata commands or other software programs.

Thanks again for everyone's help. I have other questions about psmatch2 and
as I begin to articulate them I will start a new thread.

Garth


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Ariel Linden,
DrPH
Sent: Monday, April 05, 2010 10:41 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: psmatch2-identifying matched pairs

Hi Garth,

I haven't looked at the specific lines of code in PSMATCH2, but it seems
likely to me that the algorithm runs the ATT, then ATU, then ATE last. Each
iteration will naturally change the matches, since ATT means finding
controls to match to treated, ATU means finding treated to match to
controls, and ATE looks to adjust all units to get at the population average
effect. 

To test this, try running the code in order ATT, ATU, and ATE, and review
the matches after each iteration. See if they change, and if they are
dropped after ATE.

Finally, you can email the authors of PSMATCH2. I have contacted them in the
past and found them very helpful.

Ariel 


Date: Sun, 4 Apr 2010 23:43:32 -0500
From: "Garth Rauscher" <garthr@uic.edu>
Subject: st: RE: RE: psmatch2-identifying matched pairs

Thanks to Ariel for the explanation. To estimate the average treatment
effect in the population (ATE) it makes sense that the 1:1 matching would
not be used. However, with the ATE option in PSMATCH2, the output also
includes estimates of ATT and ATU along with ATE, therefore it would seem
that the program would still need to know the matched pairs in order to
estimate ATT. Therefore I don't understand why the identified pairs change
and some appear to be "broken".

Garth Rauscher
Associate Professor of Epidemiology
Division of Epid/Bios (M/C 923)
UIC School of Public Health
1603 West Taylor Street
Chicago, IL 60612
ph: (312)413-4317
fx:  (312)996-0064
em: garthr@uic.edu
 

- -----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Ariel Linden,
DrPH
Sent: Saturday, April 03, 2010 10:04 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: psmatch2-identifying matched pairs

Garth

Per your question about ATE and dropped matches: That makes intuitive sense
since the ATE represents the average treatment effect in the population.
Therefore, you would need to have the outcome values of the entire
population, not just the subset of matched controls. As a result, you would
no longer have 1:1 matches.

Ariel

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