From cohort study (prospective) Risk of developing disease for people with known exposure compared to risk of developing disease without exposure Incidence risk of YES = b / a Incidence risk of NO = d / c Relative risk = (b / a) / (d/c) Exposed Disease YES a b NO c d Example A study is performed concering the relationship between blood transfusions and the risk of developing hepatitis C. A group of patients is studied for three years. Transfused Hepatitis C YES 595 75 NO 712 16 Incidence Group "YES" = 75/595 = .126 Incidence Group "NO" = 16/712 = .022 Relative risk = 12.6 / 2.2 = 5.7%