Re: st: how to deal with censoring at zero (a lot of zeroes) fora laboratory re

[Arnold Kester <arnold.kester@stat.unimaas.nl>]

Op 06/08/2005 05:48 PM schreef Maarten Buis:
> Arnold kester wrote:
>
>
> > If you want to get a prediction for undetectable Troponin without
> > assuming a specific value you could add a dummy variable troponin_zero =
> > (troponin == 0) and substitute (say) zero for log(troponin) when
> > troponin==0. The predicted value from this model is independent of what
> > you choose for "log(0)".
>
> Running a regression on an `imputed' missing variables and a dummy variable
> to indicate whether a variable was imputed or not will generally lead to biased
> estimates. 
However, in this case the var is not missing, it is (close to) zero, a 
valid value. The proposed model supposes a discontinuity in the response 
between detectable and undetectable.



> Both points are made in one of those convenient little green sage booklets: 
> Paul Allison (2002) Missing Data. Tousand Oaks, Sage.
>
> Hope this helps,
> Maarten
--
Met vriendelijke groet,
Arnold Kester

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