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


From   "Anders Alexandersson" <andersalex@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Number Needed to be Treated (NNT)
Date   Thu, 2 Aug 2007 16:20:05 -0400

Svend Juul and then Nick Cox wrote a small program to calculate number
needed to be treated (NNT) based on -cs- and this was one example
output:
>     NNT = 15.230769  (95% CI: 5.0701399; -15.169883)

NNT is also related to the probability that variable for first group
is larger than for second group, P(X>Y). For a reference, see Acion,
L. et al. 2006. "Probabilistic index: an intuitive non-parametric
approach to measuring the size of treatment effects". Statistics in
Medicine 25(4): 591-602. There was a follow-up with reply later on
pages 3944-3948.
It seems to me that NNT = -(1/(2P(X>Y)-1)).

Therefore, you can also calculate NNT quickly using either -ranksum-
with the porder option or Roger Newson's command -somersd- with the
tr(c) option:
. ranksum low, by(x) porder-
[output ommitted]
. di -(1/((2*0.467)-1))
15.151515

or, more precise and automated:
. somersd  x low, tr(c)
. di -(1/((2*(1-_b[low]))-1))
15.230769

This does not give a 95% confidence interval (yet) but is another way
of calculating NNT.

Anders Alexandersson
andersalex@gmail.com
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