Number needed to treat and number needed to harm are not the best way to report and assess the results of randomised clinical trials.

The inverse of the difference between rates, called the 'number needed to treat' (NNT), was suggested 20 years ago as a good way to present the results of comparisons of success or failure under different therapies. Such comparisons usually arise in randomised controlled trials and meta-analysis. This article reviews the claims made about this statistic, and the problems associated with it. Methods that have been proposed for confidence intervals are evaluated, and shown to be erroneous. We suggest that giving the baseline risk, and the difference in success or event rates, the 'absolute risk reduction', is preferable to the number needed to treat, for both theoretical and practical reasons.