The number of patients who must receive a specific therapy (or undergo a specific medical test) so that one of them will benefit. This concept is important in assessing the relative values and costs of interventions for specific illnesses. For example, to prevent one death from breast cancer, the number of patients who need annual mammography can be calculated. Similarly, the number of patients with cancer who will survive because of the use of a particular chemotherapy can be assessed. Generally, the smaller the number needed to treat, the greater the value of the intervention. This comparative information can be used to decide how to allocate resources, plan studies, or make recommendations to patients about their care.