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Re: st: Number Needed to be Treated (NNT)


From   Ronan Conroy <rconroy@rcsi.ie>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Number Needed to be Treated (NNT)
Date   Thu, 2 Aug 2007 10:11:23 +0100

On 1 Aug 2007, at 13:55, Svend Juul wrote:

    NNT = 15.230769  (95% CI: 5.0701399; -15.169883)
- the negative numbers meaning that exposure to x may prevent low
birthweight.

The negative number has the interpretation of Number Needed to Harm (with its minus sign removed), just as a negative Attributable fraction is more usually written as "Prevented fraction in the exposed/population"

Thus the confidence interval in Svend's example runs from an NNT of 5 to an NNH of 15.

The other useful statistic that I recommend is the NTN (number treated needlessly). It is the NNT-1. So if you have to treat 400 people to prevent one event, then the number treated needlessly is 399. A salutary reflection for the average doctor.


P Before printing, think about the environment
=================================
Ronan Conroy
rconroy@rcsi.ie
Royal College of Surgeons in Ireland
120 St Stephen's Green, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95





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