Attributable Risk and Population Attributable Risk (PAR) Measures

1. Core Knowledge:These are a set of statistics that estimate the impact of a causal factor.

- You should be able to distinguish between the Attributable Risk, the Population Attributable Risk and the Population Prevented Fraction.

Details

1. The

indicates the number of cases of a diseaseAttributable Riskamong exposed individualsthat can be attributed to that exposure:

Incidence(exposed) – Incidence(unexposed)(This shows how much extra disease has been caused by this exposure)1.1 This can also be expressed

as a fractionof all exposed:

[ Incidence(exposed) – Incidence(unexposed) ] ÷ Incidence(exposed).

2. The

(orPopulation Attributable Risk) indicates the number (or proportion) of cases that would not occurPopulation Attributable Fractionin a populationif the factor were eliminated (e.g. how many lives would be saved if people no longer smoked?)

(But be careful, for there are several synonyms and related concepts that can easily be confused with each other: "Population attributable risk"; "population attributable risk proportion"; "etiologic fraction"; "excess fraction")

The attributable risk in a population depends on the prevalence of the risk factor and the strength of its association (relative risk) with the disease. The formula is

PAR = P_{e}(RR_{e}-1)/[1 + P_{e}(RR_{e}-1)],

where P_{e }is the prevalence of the exposure (e.g., proportion who are overweight) and RR_{e}is the relative risk of disease due to that exposure.3. The

refers to situations where exposure to a factor is protective.Population Prevented Fraction

The prevented fraction is the proportion of the hypothetical total load of disease that has been prevented by exposure to the protective factor. The formula is

P_{e }(1-RR)4. The

refers to the number of cases attributable to an exposure. The formula isAttributable Number

AN = N_{e}(I_{e }- I_{u}),

where N_{e}is the number exposed, I_{e}is the incidence among those exposed, and I_{u}is the incidence among those unexposed to that factor.

Further details: B. Rockhill et al. Use and misuse of population attributable fractions"