st: Interpretation of sts list

[Christina M Gibson-Davis <cgibson@duke.edu>]

Hello,
I am having difficulty interpreting the "survivor function" column of sts 
list. I had thought it reported the percentage of cases that had survived 
during a particular time interval, and thus it would be possible to tell, 
by looking at the last entry, how many cases had survived without failing. 
However, by using the cancer data available through Stata (webuse cancer), 
I see that this cannot be.

If I type:
webuse cancer
stset study died
stdes

I see that there are 48 subjects, and 31 failed, for a survival rate of 35% 
(48-31=17; 17/48 = 35%).

But if I type "sts list, at(0 5 to 39)", I end up with a survival rate of 
19% (please see below).


             Beg.                      Survivor      Std.
   Time     Total     Fail             Function     Error     [95% Conf. 
Int.]
-------------------------------------------------------------------------------
      1        48        2              0.9583    0.0288     0.8435 
0.9894
      5        42        6              0.8333    0.0538     0.6943 
0.9129
      9        32        6              0.7061    0.0661     0.5546 
0.8143
     13        24        6              0.5628    0.0742     0.4060 
0.6931
     17        20        3              0.4870    0.0761     0.3326 
0.6249
     21        16        0              0.4870    0.0761     0.3326 
0.6249
     25        10        6              0.2922    0.0767     0.1543 
0.4449
     29         8        1              0.2557    0.0753     0.1247 
0.4093
     33         4        1              0.1918    0.0791     0.0676 
0.3634
     37         2        0              0.1918    0.0791     0.0676 
0.3634
-------------------------------------------------------------------------------

I don't understand why the survivor function doesn't end up at 35%, if 
overall, 35% of the cases survived.

Thank you very much in advance for your assistance.

Christina

___________________________________________

Christina Gibson-Davis
Assistant Professor of Public Policy
Center for Child and Family Policy
Terry Sanford Institute
Duke University
Box 90245
Durham NC 27708
cgibson@duke.edu
(919) 613-7364

The thing that impresses me the most about America is the way parents obey
their children.
-- King Edward VIII
__________________________________________
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