Relative Versus Absolute Risk
A relative risk compares the risk of disease among people with a particular risk factor to the risk among people without that risk factor. If the relative risk is above 1.0, then risk is higher among those with the risk factor than those without. Relative risks below 1.0 indicate a protective effect, or lower risk, associated with a particular factor.
Relative risks are useful for comparisons, but they do not provide information about the absolute amount of additional risk experienced by the group with the risk factor in question. For example, current users of combination estrogen and progestin hormone replacement therapy (HRT) have a relative risk of 1.26, or a 26 percent increased risk. Although this increased risk may seem substantial, it proves to be less so in absolute terms because of the very low risk of breast cancer among young women in general.
Among 10,000 women who have been using HRT for 5.2 years, 38 breast cancers would be expected to be diagnosed. Among 10,000 similar women who never used HRT, 30 cases would be expected over the same period. Therefore, the 26 percent increased relative risk results in an absolute risk of only 8 additional breast cancer cases per 10,000 women over a period greater than 5.2 years.
Adapted from the American Cancer Society, Breast Cancer Facts and Figures 2003-2004.1