The group exposed to treatment (left) has the risk of an adverse outcome (black) reduced by 50% (RRR = 0.5) compared to the unexposed group (right).

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 $(I_{u}-I_{e})/I_{u}$, 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 adverse event is increased by the exposure rather than decreased, term relative risk increase (RRI) is used, and computed as $(I_{e}-I_{u})/I_{u}$.^{[1]}^{[2]} If the direction of risk change is not assumed, a term relative effect is used and computed as $(I_{e}-I_{u})/I_{u}$.^{[3]}

^Szklo, Moyses; Nieto, F. Javier (2019). Epidemiology : beyond the basics (4th. ed.). Burlington, Massachusetts: Jones & Bartlett Learning. p. 97. ISBN9781284116595. OCLC1019839414.

^J., Rothman, Kenneth (2012). Epidemiology : an introduction (2nd ed.). New York, NY: Oxford University Press. p. 59. ISBN9780199754557. OCLC750986180.