Sami wrote:
I would be highly thankful to you if you let me know
the way in which sample size can be calculated for
the following problem
I am studying the occurrence of a disease between two
groups. I have an annual average event rate of 1.6 %
and a follow up period of 5 years, I want to have a
power of 80% with alpha 5% I am looking to detect a
relative risk of 3.0 associated with group1 versus
group 2 .
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Do you assume that there will be no censorings? If there
may be, you must make an assumption about the rate of
censorings. In the following I assume no censorings.
Next: Is the assumed incidence rate of 1.6% per year:
- for the non-exposed?
- for the exposed?
- a population average?
If the latter is the case, you also need an assumption
about the proportion exposed.
Assuming that the annual incidence rate of 1.6% is among the
unexposed, the five year incidence proportion is approximately
5*1.6% = 0.08 (actually it is 1-exp(-0.08) = 0.077, but that
is hardly important).
. sampsi .24 .08 , 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 = 0.2400
p2 = 0.0800
n2/n1 = 1.00
Estimated required sample sizes:
n1 = 94
n2 = 94
But are these assumptions right? There are more sophisticated
tools for sample size estimation. That is fine, but the most
important is to use realistic assumptions.
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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