Attributable risk in the population (ARpop)

The second type of question relates to the excess risk of disease in the total population attributable to exposure. This is the attributable risk in the population (ARpop) or the population-attributable risk. It is the proportion of cases in the general population that can be attributed to the exposure.

Ipop = incidence in the population

Iu = incidence among unexposed

It represents the reduction in risk we would achieve if the entire population were not exposed. It helps to identify which exposures are most relevant in the community and will yield the most benefit from public health interventions ^[1], ^[2].

Attributable fraction in the population (AFpop)

The population attributable risk can also be expressed as a percentage of the total risk in the population.

This is known as the attributable fraction in the population (AFpop):

Ipop = incidence in the population

Iu = incidence among the exposed

Table. Risk of death from speeding, Anystate, 2010

Speeding	Total drivers	No. of deaths	Risk of death per 1,000	Attributable risk (population)
Yes	2,000	100	50
No	8,000	80	10
Total	10,000	180	18	18 - 10 = 8/1,000

Speeding

This means that (if speeding causes driving-related deaths) 44% of driving-related deaths in the population can be attributed to speeding.

Table. Risk of death from drunk driving, Anystate, 2010 Drunk driving

Drunk driving	Total drivers	No. of deaths	Risk of death per 1,000	Attributable risk (population)
Yes	300	45	150
No	9,700	135	14
Total	10,000	180	18	18 - 14 = 4/1,000

Drunk driving

This means that (if drunk driving causes driving related deaths) 22% of driving related deaths in the population can be attributed to drunk driving.

AFpop can also be expressed as:

The above formula is not valid if the RR is adjusted for confounders, as is often the case. In this situation one of the following alternatives is preferable:

Pe = proportion of the population exposed

PCe = proportion of cases exposed

Ie = incidence in exposed

Iu = incidence in unexposed

RR = risk ratio

ARe = attributable risk among exposed

If the risk factor is causal, then the population-attributable risk depends on:

the strength of the association (RR) the frequency of the exposure (Pe) To have a large impact on the population, exposure must be common.

Methods are also available for dealing with multiple exposure categories for a single risk factor ^[3], and for diseases caused by multiple risk factors ^[2] ^[4].

Sometimes, diseases are the result of complex interactions between risk factors. Methods to conceptualise and clarify these interactions have been developed. These include sequential attributable fractions ^[5], ^[6], and causal pies ^[7], ^[8].

Synopsis