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Re: st: Survival function of regression Cox model postestimation
From
Maarten Buis <[email protected]>
To
[email protected]
Subject
Re: st: Survival function of regression Cox model postestimation
Date
Mon, 11 Jul 2011 08:41:08 +0200
On Fri, Jul 8, 2011 at 8:13 PM, <[email protected]> wrote:
> I know that the baseline survival function of the Cox model without
> covariates is the same as the Kaplan Meier survival function.
> However, it is unclear for me why the survival function of a Cox model with
> covariates obtained at the mean values of the covariates is different from
> baseline survival function of the Cox model without covariates.
Because the relationship between the covariates and the survivor
function is a non-linear one. Remember that applying a non-linear
transformation to a variable x and than compute the mean is not the
same as computing the mean of variable x and than apply that
non-linear transformation.
If this is new to you, try the example below:
*---------- begin example ----------
sysuse nlsw88, clear
gen ln_w = ln(wage)
sum ln_w
sum wage
di ln(r(mean))
*------------ end example ----------
Hope this helps,
Maarten
--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany
http://www.maartenbuis.nl
--------------------------
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