We could instead ask, “What proportion of the risk in exposed persons is due to the exposure?” We could then express the attributable risk as the proportion of the total incidence in the exposed group that is attributable to the exposure by simply dividing Formula 12-1 by the incidence in the exposed group, as follows:Formula 12-2
The attributable risk expresses the most that we can hope to accomplish in reducing the risk of the disease if we completely eliminate the exposure. For example, if all smokers were induced to stop smoking, how much of a reduction could we anticipate in lung cancer rates? From a practical programmatic standpoint, the attributable risk may be more relevant than the relative risk. The relative risk is a measure of the strength of the association and the possibility of a causal relationship, but the attributable risk indicates the potential for prevention if the exposure could be eliminated.
The practicing clinician is mainly interested in the attributable risk in the exposed group: For example, when a physician advises a patient to stop smoking, he or she is in effect telling the patient that stopping smoking will reduce the risk of coronary heart disease (CHD). Implicit in this advice is the physician—s estimate that the patient—s risk will be reduced by a certain proportion if he or she stops smoking; the risk reduction is motivating the physician to give that advice. Although the physician often does not have a specific value in mind for the attributable risk, he or she is in effect relying on an attributable risk for an exposed group (smokers) to which the patient belongs. The physician is implicitly addressing the question: In a population of smokers, how much of the CHD that they experience is due to smoking, and, consequently, how much of the CHD could be prevented if they did not smoke? Thus, attributable risk tells us the potential for prevention.
If all the incidence of a disease were the result of a single factor, the attributable risk for that disease would be 100%. However, this is rarely if ever the case. Both the concept and the calculation of attributable risk imply that not all of the disease incidence is due to a single specific exposure, as the disease even develops in some nonexposed individuals. Figure 12-2 recapitulates this concept.