Re: st: Failure time in stset.

[Ron�n Conroy <rconroy@rcsi.ie>]

adiallo5@worldbank.org wrote:

> Hi,
> I have a trouble knowing what exactely is the time variable
> in stset command.
> I want to compute a weibull model on the determinants of
> child survival and I have an age variable along with
> household and child characteristics.
> Is it ok to set age as the analysis time, with correcting for
> those children who are dead?
> I do not have a origin and exit dates, only the age of child and the
> interview date.
>
> su agechild
> replace agechild = agedeath if dead==1
>
> stset agechild, failure(dead) id(childid)
>  
This is the most logical way to treat age, and is the subject of an 
excellent paper by Ed Korn which points out that we often treat time 
prior to coming under observation (age) as a risk factor but time after 
coming under observation as the time variable.

Korn EL, Graubard BI, Midthune D. Time-to-event analysis of longitudinal 
follow-up of a survey: choice of the time-scale. Am J Epidemiol 1997; 
145(1):72-80.

In your example, remember that a child could die before coming under 
observation. So your origin is age zero, but children enter at the age 
when they were first observed and exit at either age of death or age of 
last observation.


> Is it normal that stsum produces missings for 25, 50 and 75 percentiles?
>  
25% of children have to die before there can be a 25%ile. This probably 
hasn't happened. Since percentiles are a matter of fact, they -stsum- 
isn't prepared to infer them. Your Weibull model could be used to infer 
them, but it would be going beyond that data, which is never a wonderful 
idea, especially when making gloomy statements about the deaths of 
children (there are many conditions characterised by a significant early 
death rate, followed by a flattening out, as those who are going to 
recover do so.)


> Best regards.
> Amadou.
>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>
>
>  

--

Ronan M Conroy (rconroy@rcsi.ie) 
Senior Lecturer in Biostatistics 
Royal College of Surgeons 
Dublin 2, Ireland 
+353 1 402 2431 (fax 2764) 
-------------------- 
Just say no to drug reps 
http://www.nofreelunch.org/

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/