Number needed to treat (NNT) is an attractive concept for clinicians, but its application to a treatment that reduces the frequency of COPD exacerbations is not straightforward. Dr Suissa1 proposes a method for calculating the NNT based on the Kaplan–Meier curve of time to first exacerbation. This approach ignores all exacerbations beyond the first one experienced by the patient and does not appropriately reflect the benefit of a treatment that reduces multiple exacerbations in the same patient.2 From a statistical viewpoint, the approach is not efficient as it does not use all the exacerbation data collected. Dr Suissa has himself previously advocated methods of analysis of exacerbations …