Our discussion in the previous chapter focused on the relative risk and on the odds ratio, which is often used as a surrogate for the relative risk in a case-control study. The relative risk is important as a measure of the strength of the association, which (as Chapter 14 will demonstrate) is a major consideration in deriving causal inferences. In this chapter, we turn to a different question: How much of the disease that occurs can be attributed to a certain exposure? This is answered by another measure of risk, the attributable risk, which is defined as the amount or proportion of disease incidence (or disease risk) that can be attributed to a specific exposure. For example, how much of the lung cancer risk experienced by smokers can be attributed to smoking? Whereas the relative risk is important in establishing etiologic relationships, the attributable risk is in many ways more important in clinical practice and public health, because it addresses a different question: How much of the risk (incidence) of disease can we hope to prevent if we are able to eliminate exposure to the agent in question?
We can calculate the attributable risk for exposed persons (e.g., the attributable risk of lung cancer in smokers) or the attributable risk for the total population, which includes both exposed and nonexposed persons (e.g., the attributable risk of lung cancer in a total population, which consists of both smokers and non-smokers). These calculations and their uses and interpretations are discussed in this chapter.