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Re: st: sample size calculation for survival data

From   "Svend Juul" <SJ@SOCI.AU.DK>
To   <>
Subject   Re: st: sample size calculation for survival data
Date   Tue, 6 Mar 2007 19:39:58 +0100

Paola wrote:
Does anybody know how to calculate sample size in the following
situation with Stata?
H0: s1=s2 (survival at 2 years) H1: one side (logrank non paramentric
test for exponential data)
Size ratio =1
S1=77,5%, s2=83,125%
Lost follow-up 10% per year
Accrual time =2 years
Total study time =4 years (2 enrollment+2 follow-up)
The follow-up time is important; the enrollment time is not.
This is a case of comparing two proportions, and you give
the assumed proportions of survivors after two years. 
The command then is:
    . sampsi 0.775 0.83125, power(.80)
    Estimated sample size for two-sample comparison of proportions
    Test Ho: p1 = p2, where p1 is the proportion in population 1
                        and p2 is the proportion in population 2
             alpha =   0.0500  (two-sided)
             power =   0.8000
                p1 =   0.7750
                p2 =   0.8313
             n2/n1 =   1.00
    Estimated required sample sizes:
                n1 =      819
                n2 =      819
Since you assume 10% lost to follow-up per year you should
probably include 1,000 persons in each group.
For immediate commands like -sampsi- I prefer to use the dialog.
It may be difficult to find, but since I know the command's name,
I could:
    . db sampsi
Hope this helps

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 

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