# risk difference

The value of the difference between the probability that an event will occur 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:** For example, if the results of a trial were that the probability of death was 25% in the control group and 10% in the experimental group, the absolute risk reduction would be 0.25 – 0.10 = 0.15.

Syn.: absolute risk reduction

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