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Re: st: Survival analysis

From   Maarten buis <>
To   stata list <>
Subject   Re: st: Survival analysis
Date   Mon, 31 Aug 2009 09:30:58 +0000 (GMT)

--- 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 ??

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).

> Would you say it is sufficient to use the graph to determine the
> "break-point" (_t>1) or should this be done in a more analytical
> fashion?

Graphical methods are just fine, especially as I doubt whether any model 
that tries to estimate this point would converge. The data contains very 
little information about this point, so even in huge datasets I doubt 
whether enough information is available to reliably estimate this point.

Hope this helps,

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


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