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Re: st: power calculation and sample size


From   "Svend Juul" <[email protected]>
To   <[email protected]>
Subject   Re: st: power calculation and sample size
Date   Sat, 7 Apr 2007 11:03:34 +0200

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 .
--------------------------------------------------------
 
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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