![cover image](https://wikiwandv2-19431.kxcdn.com/_next/image?url=https://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Illustration_of_risk_reduction.svg/640px-Illustration_of_risk_reduction.svg.png&w=640&q=50)
Risk difference
From Wikipedia, the free encyclopedia
The risk difference (RD), excess risk, or attributable risk[1] is the difference between the risk of an outcome in the exposed group and the unexposed group. It is computed as , where
is the incidence in the exposed group, and
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
. Equivalently, if the risk of an outcome is decreased by the exposure, the term absolute risk reduction (ARR) is used, and computed as
.[2][3]
![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).](http://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Illustration_of_risk_reduction.svg/320px-Illustration_of_risk_reduction.svg.png)
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.[2]