 Group exposed to a risk factor (left) has increased risk of an adverse outcome (black) compared to the unexposed group (right). In the exposed group, one third of the adverse outcomes can be attributed to the exposure (AFe = 1/3).

In epidemiology, attributable fraction among the exposed (AFe) is the proportion of incidents in the exposed group that are attributable to the risk factor. The term attributable risk percent among the exposed is used if the fraction is expressed as a percentage. It is calculated as $AF_{e}=(I_{e}-I_{u})/I_{e}=(RR-1)/RR$ , where $I_{e)$ is the incidence in the exposed group, $I_{u)$ is the incidence in the unexposed group, and $RR$ is the relative risk. It is used when an exposure increases the risk, as opposed to reducing it, in which case its symmetrical notion is preventable fraction among the unexposed.

## Synonyms

Multiple synonyms of AFe are in use: attributable fraction, relative attributable risk, attributable proportion among the exposed, and attributable risk among the exposed.

Similarly, attributable risk percent (ARP) is used as a synonym for the attributable risk percent among the exposed.

In climatology, fraction of attributable risk (FAR) is used to denote a proportion of adverse event risk attributable to the human influence on climate or other forcing factor.

## Numerical example

Example of risk increase
Quantity Experimental group (E) Control group (C) Total
Events (E) EE = 75 CE = 100 175
Non-events (N) EN = 75 CN = 150 225
Total subjects (S) ES = EE + EN = 150 CS = CE + CN = 250 400
Event rate (ER) EER = EE / ES = 0.5, or 50% CER = CE / CS = 0.4, or 40%
Variable Abbr. Formula Value
Absolute risk increase ARI EERCER 0.1, or 10%
Number needed to harm NNH 1 / (EERCER) 10
Relative risk (risk ratio) RR EER / CER 1.25
Relative risk increase RRI (EERCER) / CER, or RR − 1 0.25, or 25%
Attributable fraction among the exposed AFe (EERCER) / EER 0.2
Odds ratio OR (EE / EN) / (CE / CN) 1.5