In epidemiology, attributable fraction among the exposed (AF_{e}) 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 , where is the incidence in the exposed group, is the incidence in the unexposed group, and 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.
Multiple synonyms of AF_{e} 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]}
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 | EER − CER | 0.1, or 10% |
Number needed to harm | NNH | 1 / (EER − CER) | 10 |
Relative risk (risk ratio) | RR | EER / CER | 1.25 |
Relative risk increase | RRI | (EER − CER) / CER, or RR − 1 | 0.25, or 25% |
Attributable fraction among the exposed | AF_{e} | (EER − CER) / EER | 0.2 |
Odds ratio | OR | (EE / EN) / (CE / CN) | 1.5 |