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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   Sun, 17 Nov 2002 23:34:27 -0500

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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