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RE: st: RE: Detection of disease

From   "Carlo Georges" <>
To   <>
Subject   RE: st: RE: Detection of disease
Date   Fri, 15 Aug 2008 17:49:25 +0200

Many thanks for the replies also to Paul Seed for his in-depth explanations.
I guessed there would be a way to do the calculations in stata.

-----Original Message-----
[]On Behalf Of Ronan Conroy
Sent: Freitag, 15. August 2008 17:38
Subject: Re: st: RE: Detection of disease

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),

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

which is accessible online.

Ronan Conroy
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?)

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