Number (of Whom?) Needed to Treat (with What?)

The number needed to treat (NNT) is a widely used measure of the potential impact of a treatment or intervention, but it is often calculated and discussed in ways which oversimplify critical issues. Specifically, the NNT itself depends on the population under study and the specific form that "treatment" would take in that population. We discuss how understanding the difference between the effect of removing a harmful exposure and the effect of deploying a specific intervention to remove that harmful exposure can affect the calculation and interpretation of an NNT. Our discussion extends a previously described framework distinguishing exposure effects from population intervention effects.