Preventive fraction
Preventive fraction (PF), also called preventable fraction, is a calculation that can be derived from relative risk or odds ratio.
It may be used when an exposure seems to reduce the risk (in opposition to attributable risk percent), and gives the percentage of cases that can be prevented if a population is exposed to an intervention, compared to an unexposed population.
It can be calculated as ( 1 − relative risk ) or as ( 1 − odds ratio ).[1]
Worked example
Example 1: risk reduction | Example 2: risk increase | |||||
---|---|---|---|---|---|---|
Experimental group (E) | Control group (C) | Total | (E) | (C) | Total | |
Events (E) | EE = 15 | CE = 100 | 115 | EE = 75 | CE = 100 | 175 |
Non-events (N) | EN = 135 | CN = 150 | 285 | EN = 75 | CN = 150 | 225 |
Total subjects (S) | ES = EE + EN = 150 | CS = CE + CN = 250 | 400 | ES = 150 | CS = 250 | 400 |
Event rate (ER) | EER = EE / ES = 0.1, or 10% | CER = CE / CS = 0.4, or 40% | EER = 0.5 (50%) | CER = 0.4 (40%) |
Equation | Variable | Abbr. | Example 1 | Example 2 |
---|---|---|---|---|
EER − CER | < 0: absolute risk reduction | ARR | (−)0.3, or (−)30% | N/A |
> 0: absolute risk increase | ARI | N/A | 0.1, or 10% | |
(EER − CER) / CER | < 0: relative risk reduction | RRR | (−)0.75, or (−)75% | N/A |
> 0: relative risk increase | RRI | N/A | 0.25, or 25% | |
1 / (EER − CER) | < 0: number needed to treat | NNT | (−)3.33 | N/A |
> 0: number needed to harm | NNH | N/A | 10 | |
EER / CER | relative risk | RR | 0.25 | 1.25 |
(EE / EN) / (CE / CN) | odds ratio | OR | 0.167 | 1.5 |
EER − CER | attributable risk | AR | (−)0.30, or (−)30% | 0.1, or 10% |
(RR − 1) / RR | attributable risk percent | ARP | N/A | 20% |
1 − RR (or 1 − OR) | preventive fraction | PF | 0.75, or 75% | N/A |