Relative risks are estimated from prospective studies, which are described on slide 16. Individuals who are exposed (and not exposed) to a risk factor are identified at baseline. Overtime, those who develop the disease of interest are identified. The incidence of the disease is determined, separately, for the groups who were and were not exposed to the risk factor. The ratio of these rates is generated. If the ratio is statistically significantly different from 1.0, then the risk factor is said to be associated with the disease.

Example:
The incidence of lung cancer among smokers is 0.96/1000/yr. The incidence of lung cancer among non-smokers is 0.07/1000/yr. The relative risk associated with smoking in this population is 0.96/100/yr divided by 0/.07/1000/yr = 13.7. Therefore, smokers are ~14 times more likely to develop lung cancer than non-smokers.