In epidemiology, 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 the incidence in the exposed group, is the incidence in the unexposed group, and is the relative risk. It is a synonym of the relative risk reduction.
, where 
is the incidence in the exposed group, 
is the incidence in the unexposed group, and 
is the It is used when an exposure reduces the risk, as opposed to increasing it, in which case its symmetrical notion is attributable fraction among the exposed.
Numerical example
| 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 |
See also
References
- Aschengrau, Ann; Seage, George R. (2014). Essentials of epidemiology in public health (3rd ed.). Burlington, MA: Jones & Bartlett Learning. p. 67. ISBN 9781449657338. OCLC 826123155.
- Rothman, Kenneth J.; Greenland, Sander; Lash, Timothy L. (2012). Modern epidemiology (3rd ed.). Philadelphia: Lippincott Williams & Wilkins. ISBN 9780781755641. OCLC 169455558.