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Re: st: Interpretation of sts list


From   "Svend Juul" <SJ@SOCI.AU.DK>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Interpretation of sts list
Date   Wed, 14 Nov 2007 22:29:26 +0100

Christina wrote:
 
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.
 
---------------------------------------------------------
 
Some patients were censored, i.e., not followed
until they died or until week 39. Try the command:
 
    tab2 studytime died
 
This is a very basic thing in survival analysis; it is
explained in any biostatistical or epidemiological textbook.
 
Svend
 
________________________________________________________ 
 
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6 
DK-8000 Aarhus C,  Denmark 
Phone, work:  +45 8942 6090 
Phone, home:  +45 8693 7796 
Fax:          +45 8613 1580 
E-mail:       sj@soci.au.dk 
_________________________________________________________ 

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