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Re: st: question on survival analysis and selection correction

From   Maarten buis <>
Subject   Re: st: question on survival analysis and selection correction
Date   Tue, 28 Sep 2010 09:16:48 +0000 (GMT)

--- On Tue, 28/9/10, Gáti Annamária wrote:
> I am doing survival analysis on a graduate dataset.
> I analyze "how long graduates survive in unemployment" after graduation.
> Some find a job in 1 month, some in 3 etc, some not even in 30 month.
> There are graduates in my data who do not search for a job after 
> graduation as they already had one while doing their studies (they 
> mostly did evening courses etc.).
> I suppose there is a selection bias I should account for while I am 
> doing my survival analysis as those who search for a job after 
> graduation are mostly young, doing their studies full time and therefore 
> they are definitely different from the ones who earn their degrees while 
> working.
> Is it possible to use the Heckmann correction methods together with 
> survival analysis? That is the only method I have learnt a bit about.
> If I need to do this correction what is the way to specify it in my syntax?

The whole Heckman correction stuff critically depends on the assumption
that the residuals the selection and main equations folow a multivariate
normal distribution. With survival analysis, you cannot assume normally
distributed errors, so that cannot work.

I would just consider them two completely different populations, and just
study the full time student population. In the continental west European context that makes perfect sense: these groups face a completely different
labor market, and are in no way competitors, they also hardly meet or 
interact during their studies. 

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen


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