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The risk difference , 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 {\displaystyle I_{e}-I_{u}} , where I e {\displaystyle I_{e}} is the incidence in the exposed group, and I u {\displaystyle 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 is used, and computed as I e − I u {\displaystyle I_{e}-I_{u}} . Equivalently, if the risk of an outcome is decreased by the exposure, the term absolute risk reduction is used, and computed as I u − I e {\displaystyle 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.