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# Re: st: Survival analysis

 From Maarten buis To statalist@hsphsun2.harvard.edu Subject Re: st: Survival analysis Date Mon, 31 Aug 2009 09:52:49 +0000 (GMT)

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

http://www.maartenbuis.nl
-----------------------------------------

--- moleps islon  wrote:
> > This is the ouput I´m getting using your approach:
> >
> > n=896, failures=292
> >
> > stcox var,tvc(var) texp((_t>1)_t)
> >
> > rh
> >
> > var HR 0.64, p=0.005, CI 0.47-0.87
> >
> > t
> > var HR 1.01,p=0.001,CI 1.01-1.03
> >
> > So as far as I understand this the interpretation is
> > that the -var- is protective within the first 24hrs,
> > but detrimental afterwards ??

--- On Mon, 31/8/09, Maarten buis wrote:
> No, the coefficient in the t equation is an interaction
> effect. So from t =0 to t=1 the hazard ratio increased
> with 1%. So at t=0 the hazard ratio for var is
> 0.64/1.01=0.62. In other words, in the first 24hrs var
> was even more protective than afterwards (but only very
> little, so I doubt whether that has any practical
> relevance).

Sorry, I did not see that you turned around the inquality
sign (from < to >). So, in your case you assume that the
PH assumption holds in the first 24hrs, and that
afterwards the log hazard ratio changes linearly with time.
So, from t=0 to t=1 the hazard ratio of var is .64, and
after t=1 the hazard ratio increases by 1% every day. At
t=2 the hazard ratio of var is 1.01*.64=.646, at t=3
1.01^2*.64=.653, at t=4 1.01^3*.64=.659, etc.

To get the interpretation I gave in my previous post you
have to replace
stcox var,tvc(var) texp((_t>1)_t)

with
stcox var,tvc(var) texp((_t<1)_t)

Hope this helps,
Maarten

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