Attributable risk (AR) or risk difference is the difference between the incidence rates in exposed and non-exposed groups. In a cohort study, AR is calculated as the difference in cumulative incidences (risk difference) or incidence densities (rate difference). This reflects the absolute risk of the exposure or the excess risk of the outcome (e.g. disease) in the exposed group compared with the non-exposed group. AR is sometimes referred to as attributable risk in the exposed because it is used to quantify risk in the exposed group that is attributable to the exposure.
Population attributable risk (PAR) is different from AR. PAR estimates the proportion of disease in the study population that is attributable to the exposure. In order to calculate PAR, the incidence of exposure in the study population must be known or estimated. PAR is usually expressed as a percentage. See Risk (prospective) for more information.