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Re: st: RE: Detection of disease


From   Ronan Conroy <rconroy@rcsi.ie>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: RE: Detection of disease
Date   Fri, 15 Aug 2008 16:38:26 +0100

On 14 Aug 2008, at 16:57, On Behalf Of Carlo Georges wrote:

For example i need to detect with 95% confidence the abscence of disease
in
a population where the presumed prevalence would be 20%. How lrge a
sample
size do I need to be 95% certain that the population is free from
disease.
This is an impossible task, I think. A better approach would be to ask what the maximum disease prevalence would be to result in zero observed cases in a sample size N.

There was a lovely paper years ago in JAMA called
Hanley, J. A., & Lippman-Hand, A. (1983). If nothing goes wrong, is everything all right? Interpreting zero numerators. JAMA, 249(13), 1743-1745.

Hanley and Lippman-Hand make the point that if zero events are observed in N cases, then the upper limit is roughly 3/N. This means that even if you observe no cases in 1,000 participants, the 95% CI for the rate is zero to 3.7 per thousand (I cheated and did a -cii- on this). So you can be 95% certain that the rate is no more than 3.7 per thousand or less.

The topic is discussed in
Eypasch E, Lefering R, Kum CK, Troidl H. Probability of adverse events that have not yet occurred: a statistical reminder. BMJ. 1995 Sep 2;311(7005):619-20.

which is accessible online.

http://www.bmj.com/cgi/content/full/311/7005/619


Ronan Conroy
=================================

rconroy@rcsi.ie
Royal College of Surgeons in Ireland
Epidemiology Department,
Beaux Lane House, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
+353 (0)1 402 2764 (Fax - remember them?)
http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/

P Before printing, think about the environment




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