The concepts developed in evidence-based medicine, together with clinical experience, are the major principles on which clinical decisions are made. One of the concepts is number needed to treat (nnt), which means the number of patients who have to be treated either to prevent one additional unfavourable outcome or to achieve one additional favourable outcome. The nnt is dependent on the baseline risk (the risk of occurrence at baseline): the higher the baseline risk, the lower will be the nnt (which is more favourable). In contrast, the relative risk reduction is fairly independent of the baseline risk. nnt and number needed to harm provide numerical arguments for deciding whether a particular treatment does more good than harm.