[The number needed to treat (NNT) and the number needed to harm (NNH)].

In our previous articles we introduced the relative and absolute expressions of risk: baseline risk, relative risk (RR), relative risk reduction (RRR), absolute risk reduction (ARR), or risk difference (RD). The "number needed to xy" is another concept to describe the results of a clinical trial. Depending on the investigated problem the "number needed to xy" is the "number needed to treat", the "number needed to harm", the "number needed to vaccinate", or the "number needed to screen". In this article of our series we introduce the number needed to treat (NNT) and the number needed to harm (NNH) as a method to characterize the difference of two compared therapy options. As the other effect sizes the NNT and NNH can be used to inform patients about the expected benefit (or harm) of different therapy options.