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Re: st: power calculation and sample size
On 6 Aib 2007, at 21:01, stata_user stata_user wrote:
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 .
Aside from help calculating the sample size, I can tell you, as a
member of a research ethics committee, that an 80% power is not
adequate.
There is a simple reason for this. An 80% power gives you one chance
in five of coming to a false negative conclusion. Such false negative
conclusions waste research money, as well as discouraging further
research. And, of course, wasting money is an ethical issue, since
money wasted on a project that has a high risk of failure is money
that cannot be used to fund other research.
Ethics committees like to see power calculations for 90% and 90%
power. This gives them an idea of the effect size that the study can
reasonably be expected to detect. you will notice that Stata's
default power is 90%.
=========
Ron�n Conroy
Royal College of Surgeons in Ireland
[email protected]
+353 (0) 1 402 2431
+353 (0) 87 799 97 95
http://www.flickr.com/photos/ronanconroy
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