Julie
Not sure about all your abbreviations, but you cannot work out confidence intervals without having some indication of the number of events involved. The NNT is a proportion eg 50 needed to treat to make 1 better. If the above value was derived from research involving 100 patients in each arm whereby there were 2 additional events in the treatment arm, this would give different confidence intervals compared with if the value was derived from research involving 1000 patients in each arm whereby there were 20 additional events in the treatment arm.
When the number of events is below 5 you have to use exact statistical methods. Start to worry if the number of events is below 20. If you dont use exact methods you will be negative probabilities!
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If ARR = 0.2 with CI of -0.002 to 0.4
These give NNT of 4.6 with limits of -390 and 2.
The confidence interval for NNT stretched from -390 to - infinity then round to +infinity and back down to 2.
There is a loop round from -infinity to +infinity
Consider 1/0.0001 and 1/-0.0001 or 1/0.0000001 and 1/-0.0000001. These ARRs are very close but the NNTs are a long way apart.
Rory
Dr Rory O'Conor
Consultant Clinical Epidemiologist
Clinical Audit
Pinderfields & Pontefract Hospitals NHS Trust
Wakefield WF1 4DG, UK
Tel: +44 1924 212850
Fax: +44 1924 814864
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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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