Risk difference

Illustration of two groups: one exposed to a risk factor, and one unexposed. Exposed group has smaller risk of adverse outcome (RD = −0.25, ARR = 0.25). — The adverse outcome (dark) risk difference between the group exposed to the treatment (left) and the group unexposed to the treatment (right) is −0.25 (RD = −0.25, ARR = 0.25).

The risk difference (RD), excess risk, or attributable risk is the difference between the risk of an outcome in the exposed group and the unexposed group. It is computed as $I_{e}-I_{u}$ , where $I_{e}$ is the incidence in the exposed group, and $I_{u}$ is the incidence in the unexposed group. If the risk of an outcome is increased by the exposure, the term absolute risk increase (ARI) is used, and computed as $I_{e}-I_{u}$ . Equivalently, if the risk of an outcome is decreased by the exposure, the term absolute risk reduction (ARR) is used, and computed as $I_{u}-I_{e}$ .

The inverse of the absolute risk reduction is the number needed to treat, and the inverse of the absolute risk increase is the number needed to harm.