# Attributable fraction among the exposed

In epidemiology, attributable fraction among the exposed (AFe) is the proportion of incidents in the exposed group that are attributable to the risk factor. Term attributable risk percent among exposed is used if the fraction is expressed as a percentage.[1] It is calculated as ${\displaystyle AF_{e}=(I_{e}-I_{u})/I_{e}=(RR-1)/RR}$, where ${\displaystyle I_{e}}$ is the incidence in the exposed group, ${\displaystyle I_{u}}$ is the incidence in the unexposed group, and ${\displaystyle RR}$ is the relative risk.[2]

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,[1][3] relative attributable risk,[1] attributable proportion among the exposed,[1] attributable risk among the exposed.[4]

Similarly, attributable risk percent (ARP) is used as a synonym for the attributable risk percent among the exposed.[3]

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.[5]

## Numerical example

Example of risk increase
Experimental group (E) Control group (C) Total
Events (E) EE = 75 CE = 100 115
Non-events (N) EN = 75 CN = 150 285
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%
EquationVariableAbbr.Value
EER CER absolute risk increaseARI0.1, or 10%
(EER CER) / CER relative risk increaseRRI0.25, or 25%
1 / (EER CER) number needed to harmNNH10
EER / CERrisk ratioRR1.25
(EE / EN) / (CE / CN)odds ratioOR1.5
(EER CER) / EERattributable fraction among the exposedAFe0.2