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%
Alfa=5%
Beta=20%
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
Assumptions:
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
________________________________________________________
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: [email protected]
_________________________________________________________
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