Re: st: Interpretation of sts list

 From "Svend Juul" To 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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```