Number needed to treat (NNT): estimation of a measure of clinical benefit

The number needed to treat (NNT) is becoming increasingly popular as an index for reporting the results of randomized trials and other clinical studies. It represents the expected number of patients who must be treated with an experimental therapy in order to prevent one additional adverse outcome event (or, depending on the context, to expect one additional beneficial outcome), compared to the expected event rates under the control therapy. Although NNT is a clinically useful measure, little work has been done on its statistical properties. In this paper, alternative NNT-type measures are defined for use with discrete or continuous data. Estimators and their variances are obtained for these measures in cross-over or parallel group designs. The ideas are illustrated with data on quality of life in asthma patients.