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

From   Richard Goldstein <>
Subject   Re: st: Number Needed to be Treated (NNT)
Date   Fri, 03 Aug 2007 12:55:21 -0400

This is an interesting and, to me, new definition of Number
Needed to Harm (NNH)

I use NNH quite differently as one over the difference
in adverse event (side effect) rates and I think it is
much more useful this way (e.g., one person can both improve
and have a side effect -- or just one or, of course, neither.

I would prefer to see the situation that Ronan is talking
about as one where neither treatment is clearly better
(from just the benefit side)

Rich Goldstein

Ronan Conroy wrote:
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

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
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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