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## EVIDENCE-BASED-HEALTH@JISCMAIL.AC.UK

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Re: NNT confidence intervals; what to do while waiting for doug altman

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Fri, 9 Apr 1999 17:52:29 +0100 (GMT Daylight Time)

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 ```you ain't done anything wrong. you've determined that the CI on your ARR crosses zero: this means that your intervention may harm as well as help. that is, your results generate an NNH as well as an NNT a. the NNT goes from 2.3 to infinity (and kisses your 4.65 along the way) b. and the NNH goes from 390 to infinity c. the nifty part is that the CI on your NNT doesn't go through zero, but through both positive and negative infinity! cheers, dls ............................................................................ Prof David L. Sackett Director, NHS R&D Centre for Evidence-Based Medicine Consultant in Medicine Editor, Evidence-Based Medicine Nuffield Department of Medicine, University of Oxford Level 5, John Radcliffe Hospital, Oxford OX3 9DU, England Phone: +44-(0)1865-221320 Fax: +44-(0)1865 222901 Email: [log in to unmask] WWW: http://cebm.jr2.ox.ac.uk     As of 1 June 1999: The Kilgore Trout Research and Conference Centre at Irish Lake,                          RR #1, Markdale, Ontario, Canada N0C 1H0 ............................................................................ On Fri, 9 Apr 1999, Julie Brown wrote: > I am hoping for a little statistical guidance with the 95% CI around an > NNT. > > Here are the specifics I am working with: The CER is 0.8, the EER is > 0.585, so the ARR is 0.215. The CI around the ARR is -0.00256 to 0.433. > So far this makes sense to me. Now, the NNT = 4.65, but taking 1/ARR for > the confidence limits, I get a 95% CI on my NNT of -390 to 2.3. This > doesn't even include my value of 4.65, and doesn't intuitively make sense. > > What am I doing wrong? > > Julie Brown, MD > Pediatric Emergency Medicine Fellow, box CH-04 > Children's Hospital and Regional Medical Center > Seattle, WA 98105 > e-mail: [log in to unmask] > fax: 206 527 3892 > > > %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%```