# Measures of impact in the population   // <![CDATA[ _spBodyOnLoadFunctionNames.push("setupPageDescriptionCallout"); // ]]>

## Attributable risk in the population (ARpop)

The second type of question we may ask relates to the excess risk of disease in the total population that is 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 population

Iu = incidence among unexposed

It represents the reduction in risk we would achieve if the entire population was not exposed. It helps to identify which exposures are most relevant in the community and will yield 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 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, the 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

### Attributable risk in the population (ARpop)

• The number of cases (amount of disease) within the population that can be attributed to the exposure

• What is the risk within the population that is due to the exposure?
• Helps in determining the public health relevance of specific exposures within the whole community
• Assumes that the causal effect is entirely due to the exposure

Synonyms:

• Attributable risk (population)

### Attributable fraction in the population (AFpop)

• The proportion of cases (percentage of disease) within the population that can be attributed to the exposure

• What is the proportion of disease within the population that:

• can be attributed to the exposure?
• could be prevented if the risk factor was eliminated?
• could be prevented if everyone was exposed to the protective factor?

Synonyms:

• Attributable fraction (population)

• Population attributable fraction

• Attributable proportion (population)

• Aetiological fraction / Preventable fraction (population)

• Population attributable risk percent

## References

1. Rockhill B, Newman B, Weiberg C. Use and misuse of population attributable fractions. Am J Public Health 1998;88:15-19.
2. Coughlin SS, Benichou J, Weed DL. Attributable risk estimation in case-control studies. Epidemiol Rev 1994;16:51-64.
3. Hanley JA. A heuristic approach to the formulas for population attributable fraction. J Epidemiol Community Health 2001;55:508-14.
4. Bruzzi P, Green SB, Dyer DP, Brinton LA, Schairer C. Estimating the population attributable risk for multiple risk factors using case-control data. Am J Epidemiol 1985;122:904-14.
5. Eide GE, Gefeller O. Sequential and average attributable fractions as aids in the selection of prevention strategies. J Clin Epidemiol 1995;48:645-55.
6. Rowe A, Powell KE, Flanders WD. Why population attributable fractions can sum to more than one. Am J Prev Med 2004;26:243-9.
7. Hoffmann K, Flanders WD. Estimating the proportion of disease due to classes of sufficient causes. Am J Epidemiol 2006;164:1253-5.
8. Liao S-F, Lee W-C. Weighing the causal pies in case-control studies. Ann Epidemiol 2010;20:568-73.