st: -stsum- vs -stci-

[Constantine Daskalakis <constantine.daskalakis@mail.tju.edu>]

Hi all.

I welcome comments from anyone but this is probably for the Stata people...

I came across this issue in a real dataset I am analyzing but I have 
simplified it here to clarify the point.

Consider the following (contrived) failure-time dataset with 12 
observations (6 in each of two experimental groups). All 12 observations 
are events (ie, no censored obs):

. list group t fail

         group          t       fail
  1.         1          1         1
  2.         1          2          1
  3.         1          3          1
  4.         1          4          1
  5.         1          5          1
  6.         1          6          1
  7.         2          1          1
  8.         2          2         1
  9.         2          2         1
 10.         2          4          1
 11.         2          6          1
 12.         2          8          1

First, I -stset- my data

. stset t, failure(fail==1)

     failure event:  fail == 1
obs. time interval:  (0, t]
 exit on or before:  failure

------------------------------------------------------------------------------
       12  total obs.
        0  exclusions
------------------------------------------------------------------------------
       12  obs. remaining, representing
       12  failures in single record/single failure data
       44  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =         0
                                  last observed exit t =         8

Then, I use -stsum- and -stci- to look at median survival times for the two 
groups. But, for group 2, -stsum- gives me a median of 4 while -stci- gives 
me a median of 2. My understanding is that both should be the KM estimate 
of the median (which, from the data, should be 2). Is there a discrepancy 
in the way the survival times are computed by the two commands?

. stsum, by(group)

         failure _d:  fail == 1
   analysis time _t:  t

         |               incidence       no. of    |------ Survival time 
-----|
group    | time at risk     rate        subjects        25%       50%       75%
---------+---------------------------------------------------------------------
       1 
|           21   .2857143             6          2         3         5
       2 
|           23   .2608696             6          2         4         6
---------+---------------------------------------------------------------------
   total 
|           44   .2727273            12          2         3         6

. stci, by(group) median

         failure _d:  fail == 1
   analysis time _t:  t

             |    no. of
group        |  subjects         50%     Std. Err.     [95% Conf. Interval]
-------------+-------------------------------------------------------------
           1 |         6           3      .8660254            1          .
           2 |         6           2      .8539126            1          .
-------------+-------------------------------------------------------------
       total |        12           3      1.154701            1          6




________________________________________________________________

Constantine Daskalakis, ScD
Assistant Professor,
Biostatistics Section, Thomas Jefferson University,
125 S. 9th St. #402, Philadelphia, PA 19107
   Tel: 215-955-5695
   Fax: 215-503-3804
   Email: constantine.daskalakis@mail.tju.edu
   Webpage: http://www.kcc.tju.edu/Science/SharedFacilities/Biostatistics

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