The Numbers Needed to Treat and Harm (NNT, NNH) Statistics: What They Tell Us and What They Do Not

Research papers and research summaries frequently present information in the form of derived statistics such as the number needed to treat (NNT) and the number needed to harm (NNH). These statistics are not always correctly understood by the reader. This article explains what NNT and NNH mean; presents a simple, nontechnical explanation for the calculation of the NNT; addresses the interpretation of the NNT; considers applications of the NNT; and discusses the limitations of this statistic. The NNH is also briefly considered.