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st: Stratification in Survival Analysis


From   "Kevin Daley" <kevin.daley@mail.mcgill.ca>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Stratification in Survival Analysis
Date   Sun, 2 Dec 2007 13:46:55 -0500

Hi all,

   I would like to ask a statistical question regarding stratification in survival analysis.  

I'm running a discrete-time event-history model of time-to-drop-out on a sample of adult students.  I've decided to stratify by enrolment status (part-time or fulltime in any given person-term), as most of the effects of the other covariates seem to be different for these two groups.  I would like to know, however, whether it is necessary to restrict my data-set so that no subject contributes person-terms to both models.  This basically comes down to a general question of whether it is appropriate or advisable to stratify by a time-varying covariate.  While there are actually very few instances of subjects switching enrolment status in my sample, I have tentatively opted to censor anyone upon the first time they switch enrolment status (in the same way that a single event model deals with competing events by censoring upon the occurrence of an alternative event).  Is it necessary that one ensure any stratification in a survival analysis is done by only time invariant predict!
 ors?  If so, is there a better way of dealing with the problem than the one that I have adopted?  Thank you very much, Kevin. 

 

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