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. It is calculated as , where is the incidence in the exposed group, is the incidence in the unexposed group, and 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.
Similarly, attributable risk percent (ARP) is used as a synonym for the attributable risk percent among the exposed.
|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%|
|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|
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