# relative risk reduction

The ratio (quotient) of the risk difference for the occurrence of an event in the group exposed to a given factor and the probability that this event will occur in the group not exposed to this factor.

**Note:** Thus, if the results of a trial show that the probability of death is 25% in the control group and 10% in the experimental group, the relative risk reduction is (0.25 - 0.10) ÷ 0.25 = 0.6.

(Related concepts: absolute risk reduction, number needed to treat, and odds ratio.)