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st: Re: statalist-digest V4 #1153


From   Paul Seed <p.t.seed@qmul.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: statalist-digest V4 #1153
Date   Mon, 10 Feb 2003 19:19:51 +0000

Quoting statalist-digest <owner-statalist@hsphsun2.harvard.edu>:

> statalist-digest       Sunday, February 9 2003       Volume 04 : Number

> Paul O'Brien wrote:
> 
> ====================
> Can I calculate the sample size for an equivalence trial with binary
> outcomes.

And Philip Ryan replied with a useful program,

ending with the warning
> Remember this simple program uses normal approximations, so when dealing 
> with proportions less than about 0.2 you are on shaky ground.

to which Paul replied 

> Unfortunately I am dealing with proportions of the order of 0.01.


In fact, Philip's warning may be over-cautious.
For rare events, both the Normal approximation and the 
power depend  more on the number of events than on the 
proportion.  You are approximating more & more closely 
to a Binomial distribution.  For the numbers of events 
necessary for an equivalence study, Normal approximation is
not going to be a problem.


Using sampsi with 90% power:

p1     p2     n1     e1
.1     .2     286    28.6
.01    .02    3300   33.0
.001   .002   33444  33.4
.0001  .0002  334875 33.5

-- 
Paul T Seed       (Please note change of e-mail address)
Lecturer in Medical Statistics
Maternal & Fetal Research Unit, 
Department of Obstetrics & Gynaecology,   
GKT School of Med., KCL.

10th Floor, North Wing, 
St Thomas' Hospital, Lambeth Palace Road, 
London SE1 7EH
tel  (+44) (0) 20 7960 5889 (except Thursdays)
fax (+44) (0) 20 7620 1227
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