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st: propsensity scoring for 4 groups in observational design


From   oconnor7@u.washington.edu
To   statalist@hsphsun2.harvard.edu
Subject   st: propsensity scoring for 4 groups in observational design
Date   Mon, 4 Oct 2010 14:39:59 -0700 (PDT)

Hello, 

I'm relatively new to Stata and very new to this listerv. I run Stata 11 for Windows. I am wondering whether anyone has advice on obtaining propensity scores for 4 groups in a longitundinal, observational study. We have a sample of children who experienced a traumatic brain injury and have been divided into 3 groups based upon severity of injury. We also have an arm injury comparison group. It was recommended that I create a propensity score for being in a group by using the logit command, followed by the grouping variable and then covariates of interest. For instance: 

"logit inj_sev maxais gender age" 

After more research, it I decided to download pscore updates. However, I am worried that the grouping variable has to be dichotomous (treatment/no treatment) in order for this command to run correctly. When I enter the following command: 

"pscore  new_inj_severity2  maxais_n iss_n, pscore(prop) logit"

I receive the following error message:

Estimation of the propensity score 

outcome does not vary; remember:
                                  0 = negative outcome,
        all other nonmissing values = positive outcome

I'm not quite sure where to go from here. I do have two main questions: 

1) Is there a better command for determining the propsensity score for four groups?
2) Am I doing something else incorrect to receive teh error message above?


Any other tips for determining propensity scores is greatly appreciated. Thank you for taking the time to read and consider responding to my query. 

Best, 
Stephen


Senior Fellow
Harborview Injury Prevention & Research Center
Department of Psychiatry & Behavioral Sciences
University of Washington at Harborview Medical Center


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