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Date:         Mon, 24 Jul 2006 18:52:02 -0400
Reply-To:     Russell Callaghan <Russell_Callaghan@camh.net>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Russell Callaghan <Russell_Callaghan@camh.net>
Subject:      Propensity Score Matching
Content-Type: text/plain; charset="iso-8859-1"

Hello All,

I am conducting a study of treatment outcomes among adolescent cocaine abusers and methamphetamine abusers. The cocaine abusers and the methamphetamine abusers have a number of large differences on observed covariates, and these differences can lead to biased estimates of the treatment effects. I want to conduct a propensity score matching procedure, as outlined in D'Agostino, 1998 (Statistics in Medicine, 17, 2265-2281). Using a logistic regression procedure, I have obtained a propensity score for each individual in both groups and, then, I performed a logit transformation of the propensity scores. Now that I have the logit of propensity scores, I want to do the following steps in syntax:

1. Randomly order individuals in the methamphetamine and cocaine groups 2. Select the first methamphetamine subject 3. All cocaine subjects within a preset amount (or calliper) of the methamphetamine subject's estimated logit of the propensity score are then selected. 4. Mahalanobis distances, based on a given number of covariates, are calculated between these selected cocaine subjects and the target methamphetamine subject. 5. The closest cocaine subject and the selected methamphetamine target subject are then removed from the pool, and the process is repeated. 6. All remaining cocaine subjects are returned to the cocaine group, and these individuals are available for the next matching with the following target methamphetamine subject. 7. The procedure continues until all the methamphetamine abusers are matched.

This procedure is called the "Nearest available Mahalanobis metric matching within calipers defined by the propensity score" on page 2269 of D'Agostino, 1998.

Could someone please give me some tips on how to do this? I've been struggling over the last few days with the syntax. Thank you, Russ

Dr. Russ Callaghan, PhD Scientist Centre for Addiction and Mental Health/ Department of Public Health Sciences University of Toronto 33 Russell St. Toronto, ON M5S 2S1 Phone: 416.535.8501 ext. 6599


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