Inepidemiology,preventable fraction among the unexposed (PFu), is the proportion of incidents in the unexposed group that could be prevented by exposure. It is calculated as, where is theincidence in the exposed group, is the incidence in the unexposed group, and is therelative risk.[1][2] It is a synonym of therelative risk reduction.
It is used when an exposure reduces the risk, as opposed to increasing it, in which case its symmetrical notion isattributable fraction among the exposed.[citation needed]
| Quantity | Experimental group (E) | Control group (C) | Total |
|---|---|---|---|
| Events (E) | EE = 15 | CE = 100 | 115 |
| Non-events (N) | EN = 135 | CN = 150 | 285 |
| Total subjects (S) | ES =EE +EN = 150 | CS =CE +CN = 250 | 400 |
| Event rate (ER) | EER =EE /ES = 0.1, or 10% | CER =CE /CS = 0.4, or 40% | — |
| Variable | Abbr. | Formula | Value |
|---|---|---|---|
| Absolute risk reduction | ARR | CER −EER | 0.3, or 30% |
| Number needed to treat | NNT | 1 / (CER −EER) | 3.33 |
| Relative risk (risk ratio) | RR | EER /CER | 0.25 |
| Relative risk reduction | RRR | (CER −EER) /CER, or 1 −RR | 0.75, or 75% |
| Preventable fraction among the unexposed | PFu | (CER −EER) /CER | 0.75 |
| Odds ratio | OR | (EE /EN) / (CE /CN) | 0.167 |