In epidemiology, the relative risk reduction (RRR) or efficacy is the relative decrease in the risk of an adverse event in the exposed group compared to an unexposed group. It is computed as ${\displaystyle (I_{u}-I_{e})/I_{u}}$, where ${\displaystyle I_{e}}$ is the incidence in the exposed group, and ${\displaystyle I_{u}}$ is the incidence in the unexposed group. If the risk of an adverse event is increased by the exposure rather than decreased, the term relative risk increase (RRI) is used, and it is computed as ${\displaystyle (I_{e}-I_{u})/I_{u}}$.^[1][2] If the direction of risk change is not assumed, the term relative effect is used, and it is computed in the same way as relative risk increase.^[3]

• Population Impact Measures
• Vaccine efficacy