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st: -stsum- vs -stci-


From   Constantine Daskalakis <constantine.daskalakis@mail.tju.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: -stsum- vs -stci-
Date   Tue, 19 Nov 2002 02:16:05 -0500

I am having trouble posting to the list -- I tried to post this yesterday but I haven't seen it appear... so, I'm trying once more.

I have noticed a slight discrepancy in the results from -stsum- and -stci- in failure-time data. Suppose that I have the following survival dataset with a total of 12 observations (6 each in 2 treatment groups), which are all events:


. list

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 11 1
8. 2 12 1
9. 2 12 1
10. 2 14 1
11. 2 15 1
12. 2 16 1

. 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
101 total analysis time at risk, at risk from t = 0
earliest observed entry t = 0
last observed exit t = 16


I am interested in finding out the median survival time for each group.
-stsum- gives 3 and 14, but -stci- gives 3 and 12.
Looking at the data, the median for the second group appears to be 12.
My understanding is that both commands should be using the Kaplan-Meier and thus should come up with the same answer. Or not? Have I missed something?


. 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 | 80 .075 6 12 14 15
---------+---------------------------------------------------------------------
total | 101 .1188119 12 3 6 14

. 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 .75 1 .
2 | 6 12 .75 11 .
-------------+-------------------------------------------------------------
total | 12 6 5.196152 2 14




________________________________________________________________

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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