Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 208

which the case-patients came is not known. As a result, risks, rates, risk ratios, or rate ratios can-
not be calculated from the typical case-control study. However, you can calculate an odds ratio and
interpret it as an approximation of the risk ratio, particularly when the disease is uncommon in the
population.

8.7 MEASURES OF PUBLIC HEALTH IMPACT
A measure of public health impact is used to place the association between an exposure and an out-
come into a meaningful public health context. Whereas a measure of association quantifies the rela-
tionship between exposure and disease, and thus begins to provide insight into causal relationships,
measures of public health impact reflect the burden that an exposure contributes to the frequency
of disease in the population. Two measures of public health impact often used are the attributable
proportion and efficacy or effectiveness .

8.7.1 a ttributable p roportion
The attributable proportion, also known as the attributable risk percent , is a measure of the public
health impact of a causative factor. The calculation of the measure assumes that the occurrence of
disease in the unexposed group represents the baseline or expected risk for the disease. It further
assumes that if the risk of disease in the exposed group is higher than the risk in the unexposed
group, the difference can be attributed to the exposure. Thus, the attributable proportion is the
amount of disease in the exposed group attributable to the exposure. It represents the expected
reduction in disease if the exposure could be removed (or never existed).

8.7.1.1 Method for Calculating Attributable Proportion
Attributable proportion is calculated as follows:

Risk forexposed group − risk for unexposed g roup

×100%

Risk forexposed group

Attributable proportion can be calculated for rates in the same way.

■ EXAMPLE 8.24
Problem: In another study of smoking and lung cancer, the lung cancer mortality rate among non-
smokers was 0.07 per 1000 persons per year (Doll and Hill, 1950). The lung cancer mortality rate
among persons who smoked 1 to 14 cigarettes per day was 0.57 lung cancer deaths per 1000 persons
per year. Calculate the attributable proportion.

Solution:

Attributable proportion = [(0.57 - 0.07)/0.57] × 100% = 87.7%

DID YOU KNOW?

Appropriate use of attributable proportion depends on a single risk factor being responsible
for a condition. When multiple risk factors may interact (e.g., physical activity and age or
health status), this measure may not be appropriate.