Attributable Risk - Diversity of Interpretation

Diversity of Interpretation

There is some variation in how the term is used.

The term population attributable risk (PAR) has been described as the reduction in incidence that would be observed if the population were entirely unexposed, compared with its current (actual) exposure pattern. In this context, the comparison is to the existing pattern of exposure, not the absence of exposure.

There is some ambiguity in terminology. Population attributable risk is often simply called "attributable risk" (AR), and the latter term is most often associated with the above PAR definition. However, some epidemiologists use "attributable risk" when referring to the excess risk, also called the risk difference or rate difference.

Greenland and Robins distinguished between excess fraction and etiologic fraction in 1988.

• Etiologic fraction is the proportion of the cases that the exposure had played a causal role in its development.

It is defined as:

where:

EF = Etiologic fraction

N_e = Number of exposed individuals in a population that develop the disease

N_n = Number of unexposed individuals in the same population that develop the disease.

• Excess fraction, however, is the proportion of the cases that occurs among exposed population that is in excess in comparison with the unexposed.

All etiologic cases are excess cases, but not vice versa. From the standpoint of both law and biology it is important to measure the etiology fraction. In most epidemiological studies, PAR measures only the excess fraction. (Larger than etiologic fraction)