# Medicine: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. The term attributable risk percent among the 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]

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).

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] and 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 = 100 175
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%
Equation Variable Abbr. Value
EER − CER absolute risk increase ARI 0.1, or 10%
(EER − CER) / CER relative risk increase RRI 0.25, or 25%
1 / (EER − CER) number needed to harm NNH 10
EER / CER risk ratio RR 1.25
(EE / EN) / (CE / CN) odds ratio OR 1.5
(EER − CER) / EER attributable fraction among the exposed AFe 0.2