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st: stci question


From   "Salah Mahmud" <salaheddin.mahmud@mcgill.ca>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: stci question
Date   Wed, 11 Jun 2003 13:57:57 -0400

I'm using stci to estimate median survival time from the following
(multiple-record) dataset.

After:
stset `datevar', id(id)  f(`var'f=2) sc(`scale')  origin(`var'f=1)  

                id:  id
     failure event:  hpvhf == 2
obs. time interval:  (vdate[_n-1], vdate]
 exit on or before:  failure
    t for analysis:  (time-origin)/30.4
            origin:  hpvhf==1

------------------------------------------------------------------------
------
    12596  total obs.
     9925  ignored because never entered
     1582  obs. end on or before enter()
      581  obs. begin on or after (first) failure
------------------------------------------------------------------------
------
      508  obs. remaining, representing
      351  subjects
      288  failures in single failure-per-subject data
 3644.836  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =         0
                                  last observed exit t =  62.13816

. stci, median

         failure _d:  hpvhf == 2
   analysis time _t:  (vdate-origin)/30.4
             origin:  hpvhf==1
                 id:  id

             |    no. of 
             |  subjects         50%     Std. Err.     [95% Conf.
Interval]
-------------+----------------------------------------------------------
---
       total |       351    7.368421      1.110286      6.21711
10.8224

Does anybody know why the CI interval (6.2-10.8) is not balanced around
the median (7.4)? The data came from a cohort study with multiple
follow-up visits (every 4 months).

Many thanks,

Salah Mahmud



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