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RE: st: stcox after psmatch2

From   "George Gunapal Pradeep Paul (NHGHQ)" <>
To   "" <>
Subject   RE: st: stcox after psmatch2
Date   Fri, 11 May 2012 09:35:43 +0000

Thanks Ariel, Steve and Joerg for your suggestions and references. To clarify more,

I am comparing a group of Chronic Obstructive Pulmonary Disease (COPD)patients enrolled in a disease management program and a control group selected from a administrative database. There is an enrolment criteria for enrolling patients into the program, but however not all COPD patients are enrolled.

I sampled control cases from hospital administrative database, i.e. COPD patients who fulfilled the COPD disease management program's enrolment criteria but where not enrolled into the program. Now that I have program patients and controls, I did matching to adjust for the difference in characteristics on who gets enrolled into the program. For eg. If younger, more healthier patients are more often enrolled in the COPD disease management program, then we can expect them to have better outcomes (lower mortality) than the control group.

My hypothesis: No difference is mortality rate (expressed in person years) between the patients enrolled in a COPD program and controls who were not enrolled in the program.

Other things to consider, multiple records per patient and the follow time is different between the program patients and controls.

Steve & Joerg: Since follow up period is different for these two groups, hence i thought of doing cox regression to adjust for the effect of follow up time on mortality on the matched program patients and controls. Kindly let me know your thoughts.

I used the following code for generating pscore and matching

xi:pscore Status age i.gender_coded i.Race_coded i.Hospital_coded i.Patient_class_coded i.Asthma i.copd i.Stroke i.chd i.hf i.Dyslipidaemia i.Obesity if PrimaryFirst1==1, pscore(mypscore) blockid(myblock) comsup

xi:psmatch2 Status if PrimaryFirst1==1, pscore(mypscore) outcome(i.Mortality_resp_system) caliper(.001) noreplace neighbor(1)

And after matching, I identified pair count using the following code
gen pair = _id if _treated==0
replace pair = _n1 if _treated==1
bysort pair: egen paircount = count(pair)

I used one matched control per treated, I selected paircount=2 and did stcox regression. Am i doing it the right way?

Thank you for your time and greatly appreciate your help.

Best regards,

-----Original Message-----
From: [] On Behalf Of Ariel Linden, DrPH
Sent: Thursday, May 10, 2012 11:52 PM
Subject: re: st: stcox after psmatch2

I am not sure what problem your facing? -psmatch2 (a user written program found on ssc) will gives you a field called _nn that tells you who the matched control is. BTW, you didn't give details of which matching approach you used or how many controls. If you used more than one matched control per treated, you'll need the _weight variable as well.

You then run your stcox model on only those patients and their matched controls (1:1), and if you have 1:k matches, you also include the _weight in the model...

I hope this helps


Date: Thu, 10 May 2012 05:33:22 +0000
From: "George Gunapal Pradeep Paul (NHGHQ)"
Subject: st: stcox after psmatch2

Dear all,
I am comparing a group of Chronic Obstructive Pulmonary Disease (COPD)patients enrolled in a disease management program and a control group.
I calculated pscore and did psmatch2 using c:\ado\plus\p\psmatch2.ado *!
version 4.0.5 18apr2012 E. Leuven, B. Sianesi. I used psmatch2 to adjust for their baseline differences and now I would like to do a cox regression on the matched program patients and controls to see if there is a difference in time to death between the two groups.

I would like to know how to run stcox after psmatch2 , thereby identifying the matching sequence for the cox regression. I tried looking for answers for this but found only one unanswered relevant question which could be found at

Kindly help.

Thank you,

Best regards,

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