# Swots' Corner:

NNTs and Confidence Intervals

### Why Bother?

NNT values without confidence intervals may be better than nothing, but they do not tell you how likely the values, or 'point estimates', are to be true. A p value tells you that a result is seldom likely to occur by chance (less than 1 or 5% of the time). A confidence interval can tell you where the true value is most likely to be (more than 90 or 95% of the time). "You can be 95% certain that the truth is somewhere inside a 95% confidence interval". [1]

### Calculating NNT

The NNT is the reciprocal of the absolute risk reduction.

### Calculating Confidence Intervals

The pukka method is to "invert and exchange the limits of a 95% CI for the ARR" [2]. The calculation using the confidence interval derivation for proportions is:

### When not to

If the odds ratio for the result is not significant (lower odds ratio confidence interval ≤ 1), then it is unwise to bother with the confidence intervals for the NNT - see Mulrow's excellent paper [4] featured in Bandolier 15 .

### References

1. Sackett DL, Haynes RB, Guyatt GH, Tugwell P. Clinical Epidemiology: a basic science for clinical medicine. Boston: Little, Brown, 1991.
2. Cook RJ, Sackett DL. The number needed to treat: a clinically useful measure of treatment effect. British Medical Journal 1995; 310: 452-4.
3. Gardner MJ, Altman DG. Statistics with confidence. London: British Medical Journal, 1989.
4. CD Mulrow, JA Cornell, CR Herren, A Kadri, L Farnett, C Aguilar. Hypertension in the elderly. Implications and generalizability of randomized controlled trials. Journal of the American Medical Association 1994 272:1932-8.

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