Yes, there is a way to make NNT and its CIs symmetric, that is to
convert the values back to their originals, ie., absolute risk
difference (ARR). Douglas Altman (BMJ 1998;317:1309-1312) suggested
to present NNT and CIs by relabelling ARR scale. From there you can
see a symmetric graph but asymmetric label just like log value
conversion.
Weiya
> When calculating the NNT for a treatment that is barely statistically
> significant and has a large NNT, the confidence interval is wide and very
> asymmetrical. For example in women with prior vertebral fractures and
> femoral neck BMD at least 2.1 standard deviations below young normal, the
> NNT with alendronate for 3 years to prevent one hip fracture =91 (95% CI
> 59-4545)
>
> I assume that if I were to able to calculate a mean NNT, it would be higher
> than 91, perhaps 200-300. Is their a formula for recalulating the NNT to
> adjust for this asymmetry? and is this a reasonable thing to do?
>
> Thanks
> Mick Braddick
> Clinical Epidemiologist
> Group Health Cooperative
> Seattle
> USA
>
>
> _______________________________________________________________
> Get Free Email and Do More On The Web. Visit http://www.msn.com
>
>
**************************************************
Dr. W Y Zhang
Centre for Evidence-Based Pharmacotherapy
Department of Pharmaceutical Sciences
Aston University
Aston Triangle
Birmingham B4 7ET
UK
Tel: +44 (0)121 359 3611 x5535
Fax: +44 (0)121 359 0733
Email: [log in to unmask]
http://www.aston.ac.uk/pharmacy/cebp/
**************************************************
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|