If an exposure to a risk leads to an increased rate of ill health outcome, the absolute effect of exposure is measured by the difference between rate of occurrence in the exposed group (RE) and the rate occurrence in the unexposed group (RU). The difference in these rates, measures the rate attributable to exposure to the risk.
Attributable risk rate = RE – RU
Following from this;
Attributable Fraction = (RE – RU )/ RU
Which is the same as;
Attributable Fraction = (RR-1)/RR
where RR = relative risk, the result obtained from most epidemiological studies).
For example, a RR of 2 gives an attributable fraction of 50%.
For example, a RR of 3 gives an attributable fraction of 66%.
It is quite possible for an individual to have a small absolute risk but quite a significant relative risk. Normally, risk management should concentrate on attributable risk.
The disease rate among an exposed group is 20 per 1000 and the disease rate among in an unexposed group is 10 per 1000. The attributable fraction is clearly 50% since 10 out of 20 cases could be attributed to work. That is, the chances that each individuals disease was caused by exposure, is 50%.
Risk management policy would need to consider the cost/benefit ratio of preventing/mitigating/compensating the 10 cases attributable to the exposure in question. The potential value of attributable risk (and epidemiology) to insurance should be clear.
Regulatory bodies do not generally publish their threshold attributable risk criteria, but where these are known, they tend to fall between 1 in 10,000 and 1 in a million per year.