Risk assessment is based on comparing disease rates (either prevalence, cumulative incidence, or incidence density, depending on context) in an "exposed" and "unexposed" population. Let R1 represent the rate of disease in the exposed group and let R0 represent the rate of disease in the unexposed group. Thereby, the relative risk (RR) = R1 / R0 and the risk difference (RD) = R1 - R0.
Relative risk quantifies the strength of an association. Relative risks of approximately 1 suggests no association
between the exposure and disease. Relative risks greater than 1 suggest a positive association, and
relative risks less than 1 suggest a negative (protective) association. The larger the relative risk, the
stronger the association between the exposure and disease.
Risk difference quantifies risk in absolute terms. The larger the risk difference, the greater the risk per population unit. A risk difference of approximately 0 indicates no association between the exposure and disease. Positive risk differences quantify potential risks and negative risk differences quantify benefits.