A phrase used in medical contexts. Suppose past results show that treatment j (j=1, 2) is successful for a proportion p_j of patients. Suppose p₁>p₂. On average, with N patients, treatment 1 will cure N(p₁−p₂) more patients than treatment 2. Thus the number needed to treat so that one extra patient is successfully treated is M, given byIf treatment 1 costs an amount k per patient more than treatment 2, then one interpretation of the above is that it will be worth using treatment 1 rather than treatment 2 only if treating a patient successfully has a value greater than kM.