Mean difference (a), small overlap (b), and big overlap (c). Suppose we are here interested in two groups. Conventionally we compare the average health of these two groups (Figure 2a). But the same average health could come from different distributions (Figures 2b and 2c). Although Figures 2b and 2c have the same average health, the overlap between groups is greater in Figure 2c than Figure 2b. A greater overlap indicates that the group characteristic does not have much effect on the way health is distributed. These figures are not based on actual distributions and used only for illustrative purposes.