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Definitions and Assumptions

The attributable fraction (AF) is used several ways in the literature (Rothman and Greenland, 1998). We use the term in a way most relevant to compensation by the Department of Veterans Affairs (VA), namely as the proportion of disease in an exposed group that can be attributed to the exposure. This report uses the terminology service-attributable fraction (SAF) when the exposed group is a military population. We begin with two simplifying assumptions: (1) exposure produces new cases of the disease that would not have occurred otherwise, and (2) the additional RR from exposure is stable over age and across subgroups within the population of exposed veterans. Later we discuss complications that might occur when these assumptions do not hold. Under these two assumptions, the AF is interpreted as the probability that among the exposed people with the disease, their disease has actually been caused by the exposure.

Crucial Properties of the AF

In applying the AF, there are two key properties. First, it is not a statement about whether the exposure is able to cause the disease. In calculating the AF, we take as given that the exposure does, in fact, cause the disease. However, even among exposed persons, the exposure does not necessarily cause all cases of the disease—most diseases have many possible causes. When an exposed person gets the disease, the chance that the disease is caused by the exposure is almost certainly less than one. The AF represents this probability.

The second important aspect of the AF is that it cannot specifically tell us which exposed people have their disease because of the exposure. All the AF can provide is an estimate of the average probability for all exposed persons. We can refine this estimate in various ways (e.g., by age or levels of exposure), but even with perfect information it is seldom, if ever, possible with current methods to identify which particular cases of a disease with multiple causes were caused by the exposure and which were not.

Estimating the AF

An AF is based on an estimation of RR, which is the ratio of disease risk among exposed persons compared to the risk among otherwise similar, but unexposed persons. RR is the most common expression of disease risk in epidemiologic studies. As discussed in Chapter 7, odds ratios from case-control studies approximate the RR.

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