Michael
I think that to perform a test *at a particular time*, as opposed to
an overall (logrank style) test, using the K-M estimator you need to
(i) estimate the S(t) and the Greenwood variance for each group at
the chosen time
(ii) perform a z test
Step (i) can be done by sts list:
. sts list , by(drug) at(0,2)
failure _d: died
analysis time _t: studytime
Beg. Survivor Std.
Time Total Fail Function Error [95%
Conf. Int.]
-------------------------------------------------------------------------------
drug=1
0 0 0 1.0000 . . .
2 18 3 0.8500 0.0798 0.6038 0.9490
drug=2
0 0 0 1.0000 . . .
2 0 0 1.0000 . . .
drug=3
0 0 0 1.0000 . . .
2 0 0 1.0000 . . .
-------------------------------------------------------------------------------
Note the syntax of the -at()- option. Had I just put -at(2)- Stata
would have given me its 2 chosen times, not necessarily t=2. (Try
it and see!)
Then you can retrieve S(t=2) for each of the two groups of interest
and the SEs and thus calculate the Greenwood estimate of the
variance of the difference in S(t). The z test follows...
Step (ii):
z = (S1(t=2) - S2(t=2)) / [ sqrt(Var(S1(t=2) + Var(S2(t=2))]
where Var(S(t=2) is the square of the SE for the respective group at
the desired time.
whence you can use the -normal()- function to get your P value.
Doesn't look like your particular data (at t=2) will support this
test - we only get an estimate for drug=1. But the above is the
general idea. Someone else may know of a more direct way.
Phil
At 11:33 AM 3/11/2008, you wrote:
Thanks Phil.
Is there a way to use -sts list- to statistically compare the
2-year survival rates between two groups? I notice that the
-compare- option simply places them next to each other without a
statistical test.
Michael
Michael
use the -noadjust- option on -ltable-
from -help ltable- :
noadjust suppresses the actuarial adjustment for deaths and
censored observations. The default is to consider the
adjusted number at risk at the start of the interval to be
total at the start minus (the number dead or censored)/2.
If noadjust is specified, the number at risk is simply the
total at the start, corresponding to the standard
Kaplan-Meier assumption. noadjust should be specified
when using ltable to list results corresponding to those
produced by sts list.
Phil
At 09:51 AM 3/11/2008, you wrote:
Dear Statalist members,
I had the impression that both -ltable- and -sts list- would list
the survivor function per failure period. However, they give
slightly different answers. In the example below, for drug3
-ltable- shows survival at 24 months as 77.14%, whereas -sts
list- says it's 64.29%
sysuse cancer.dta, clear
sts list, by(drug)
ltable _t died, by(drug)
Could anyone help me to clarify this?
--
Best wishes,
Michael McCulloch
Pine Street Foundation
124 Pine St., San Anselmo, CA 94960-2674
Tel: (415) 407-1357
Fax: (415) 485-1065
[email protected]
www.pinestreetfoundation.org
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Philip Ryan
Professor,
Discipline of Public Health
Director, Data Management & Analysis Centre
Associate Dean (IT)
Faculty of Health Sciences
postal address:
Discipline of Public Health
Mail Drop DX650 511
University of Adelaide 5005
South Australia
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Bice Building
Royal Adelaide Hospital
North Terrace
Adelaide
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fax +61 8 8223 4075
http://www.public-health.adelaide.edu.au/
CRICOS Provider Number 00123M
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