What does risk difference measure in epidemiology?

Risk difference measures the actual difference in risk between two groups, specifically comparing the incidence of disease in an exposed group to that in an unexposed group. This metric provides insight into the additional risk attributed to exposure to a certain factor. For example, if the risk of a disease in the exposed group is 10% and in the unexposed group is 5%, the risk difference would be 5%. This indicates that an additional 5% of individuals in the exposed group are affected by the disease compared to those not exposed.

This measure is particularly useful in epidemiological studies because it helps quantify the impact of a specific exposure on disease occurrence and can guide public health interventions and risk communication. Understanding risk difference allows researchers and health practitioners to see not just whether an exposure increases risk (as relative risk would), but by how much it does so.