Julie,
The short answer is that since the CI around your ARR incudes 0, the CI
around the NNT includes infinity.
Douglas Altman has written an excellent paper on the subject: "Confidence
intervals for the number needed to treat" BMJ 1998;317:1309-12.
______________________________________________
Eric Harvey, PharmD, MBA, BCPS
Child Health Institute, Evidence-Based Medicine Project
146 North Canal Street, Suite 300
Seattle, WA 98103-8652
University of Washington Campus Mailbox: 358853
office: (206) 616-1203
fax: (206) 543-5318
e-mail: [log in to unmask]
____________________________________________
----- Original Message -----
From: Julie Brown <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, April 09, 1999 9:28 AM
Subject: NNT confidence intervals
> 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
>
>
>
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