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st: AFT Models and time origin


From   Marcello Pagano <pagano@hsph.harvard.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: AFT Models and time origin
Date   Wed, 09 Aug 2006 11:52:04 -0400

Megan Andrew wrote:

*From:* Megan Andrew <mailto:mandrew@ssc.wisc.edu>
*To:* statalist@hsphsun2.harvard.edu <mailto:statalist@hsphsun2.harvard.edu>
*Sent:* Tuesday, August 08, 2006 3:29 PM
*Subject:* AFT Models and time origin

I'm estimating Accelerated Failure Time (AFT) models with a log-logistic distribution to analyze the time to various life course events in the presence of a particular treatment. Cleves et al. (2004) note that the coefficients in an AFT model are sensitive to when the origin of time is set. Not surprisingly, setting analysis time to begin at different dates changes the results from my own models. Though it is clear that setting analysis time to begin at different dates changes model results, it is not exactly clear to me why. Does anyone have thoughts on the matter or can anyone point me to technical journal articles discussing this characteristic of the AFT model? I'd like to make a more informed decision as to when I should set the origin of analysis time in my models.
Thanks,
Megan Andrew
University of Wisconsin - Madison
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