Preventable fraction among the unexposed

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.[1][2] It is a synonym of the relative risk reduction.

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

  Example of risk reduction
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%
CER - EERabsolute risk reduction ARR0.3, or 30%
(CER - EER) / CER relative risk reduction RRR 0.75, or 75%
1 / (CER EER)number needed to treatNNT3.33
EER / CERrisk ratioRR0.25
(EE / EN) / (CE / CN)odds ratioOR0.167
(CER - EER) / CERpreventable fraction among the unexposedPFu0.75

See also


  1. 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.
  2. Rothman, Kenneth J.; Greenland, Sander; Lash, Timothy L. (2012). Modern epidemiology (3rd ed.). Philadelphia: Lippincott Williams & Wilkins. ISBN 9780781755641. OCLC 169455558.
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