What is Attributable Risk?

Legal Definition
In epidemiology, attributable risk or excess risk is the difference in rate of a condition between an exposed population and an unexposed population. Attributable risk is mostly calculated in cohort studies, where individuals are assembled on exposure status and followed over a period of time. Investigators count the occurrence of the diseases. The cohort is then subdivided by the level of exposure and the frequency of disease is compared between subgroups. One is considered exposed and another unexposed. The formula commonly used in Epidemiology books for Attributable risk is Ie - Iu = AR, where Ie = Incidence in exposed and Iu = incidence in unexposed. Once the AR is calculated, then the AR percent can be determined. The formula for that is 100*(Ie - Iu)/Ie .

Population attributable risk (PAR) is the reduction in incidence that would be observed if the population were entirely unexposed, compared with its current (actual) exposure pattern. The concept was first proposed by Levin in 1953.
-- Wikipedia
Legal Definition
A risk to health that occurs due to exposure.