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RE: st: Adjusted Kaplan-Meier curves


From   "Kieran McCaul" <Kieran.McCaul@uwa.edu.au>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Adjusted Kaplan-Meier curves
Date   Tue, 9 Jun 2009 08:00:38 +0800

The curves generated after a Cox model are always going to be parallel because the Cox model assumes proportional hazards.  If your Kaplan-Meier curves are crossing, this could indicate that the hazards are not proportional.  It depends where the cross-over occurs.  They will often cross-over towards the end of follow-up, but that's usually because the data is getting sparse and the survival estimates are becoming a bit erratic.  

If the cross-over occurs at a time point where you still have a reasonable amount of data, then you need to check the proportionality assumption in the Cox model.

______________________________________________
Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2701
Fax: (08) 9224 8009
email: Kieran.McCaul@uwa.edu.au
http://myprofile.cos.com/mccaul 
http://www.researcherid.com/rid/B-8751-2008
______________________________________________
Man is a credulous animal, and must believe something; in the absence of good grounds for belief, 
he will be satisfied with bad ones. Bertrand Russell 

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Ricardo Ovaldia
Sent: Tuesday, 9 June 2009 7:33 AM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Adjusted Kaplan-Meier curves

Thank you Maarten.

That is what exactly what I did, except that I used the -at()- option to plot one curve for each drug. I used the dummies to do that such that for example for drug=1 it would be -at(_Idrug_2=0 _Idrug_3=0)- and so on. However the curves are parallel, with jumps at every death and do not look like the unadjusted curves.

Ricardo.
 
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


--- On Mon, 6/8/09, Maarten buis <maartenbuis@yahoo.co.uk> wrote:

> From: Maarten buis <maartenbuis@yahoo.co.uk>
> Subject: Re: st: Adjusted Kaplan-Meier curves
> To: statalist@hsphsun2.harvard.edu
> Date: Monday, June 8, 2009, 11:12 AM
> 
> --- On Mon, 8/6/09, Ricardo Ovaldia wrote:
> > I have been asked to plot Kaplan-Meier curves adjusted
> for
> > covariates, such as age, gender, race. 
> > My thought was to use -stcox- to adjust and then plot
> the
> > adjusted survival using -stcurve-. 
> > But I am not sure I am doing this correctly. The KM
> curves
> > plotted with -sts graph,by()- crossover, but those
> plotted
> > with -stcurve- do not and also they have a lot more
> steps
> > than the original curves. Any ideas?
> 
> Sounds like a reasonable strategy to me. I don't have any
> concrete ideas, except that I added the example below of
> how I would do this. Maybe you can see something in my 
> example that is different from what you did (no guarantees
> that what did is better than what you did though).
> 
> *--------- begin example --------
> sysuse cancer, clear
> stset studytime, failure(died)
> xi: stcox i.drug age, basesurv(S)
> stcurve, survival
> *--------- end example ----------
> 
> Hope this helps,
> Maarten
> 
> -----------------------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
> 
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
> 
> 
>       
> 
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