Re: st: RE: Detection of disease

[Ronan Conroy <rconroy@rcsi.ie>]

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
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http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/

P    Before printing, think about the environment




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