In this example, we know that the incidence among the smokers is 28.0 per 1,000 and the incidence among the nonsmokers is 17.4 per 1,000. However, we do not know the incidence in the total population. Let us assume that, from some other source of information, we know that the proportion of smokers in the population is 44% (and therefore the proportion of nonsmokers is 56%). The incidence in the total population can then be calculated as follows:
(We are simply weighting the calculation of the incidence in the total population, taking into account the proportion of the population that smokes and the proportion of the population that does not smoke.)
So, in this example, the incidence in the total population can be calculated as follows:
We now have the values needed for using Formula 12-3 to calculate the attributable risk in the total population:Formula 12-3
What does this tell us? How much of the total risk of CHD in this population (which consists of both smokers and nonsmokers) is attributable to smoking? If we had an effective prevention program (smoking cessation) in this population, how much of a reduction in CHD incidence could we anticipate, at best, in the total population (of both smokers and nonsmokers)?
If we prefer to calculate the proportion of the incidence in the total population that is attributable to the exposure, we can do so by dividing Formula 12-3 by the incidence in the total population as in Formula 12-4 :Formula 12-4
Thus, 21.3% of the incidence of CHD in the total population can be attributed to smoking, and if an effective prevention program eliminated smoking, the best that we could hope to achieve would be a reduction of 21.3% in the incidence of CHD in the total population (consisting of both smokers and nonsmokers).
Attributable risk is a critical concept in virtually any area of public health and in clinical practice, in particular in relation to questions regarding the potential of preventive measures. For example, Mokdad and colleagues[1] estimated the actual causes of death in the United States in 2000. These estimates used published data and applied attributable risk calculations as well as other approaches. Their estimates are shown in Figure 12-3 . The authors reported that tobacco and diet-activity patterns accounted for 33% of all deaths.