a type of measure of treatment effect that compares the probability of a type of outcome in the treatment group with that of a control group, i.e.: (Pc - Pt) ¸ Pc. For instance, if the results of a trial show that the probability of death in a control group was 25% and the probability of death in a control group was 10%, the relative risk reduction would be: (0.25 - 0.10) ¸ 0.25 = 0.6. (See also absolute risk reduction, number needed to treat, and odds ratio.)