Calculation of lifetime relative risks from summary cohort data and application to calculation of attributable fractions.

In cohort studies, relative risks are estimated by comparing eventual disease risk in individuals exposed to the risk factor at baseline with similar unexposed individuals. However, such relative risk estimates intrinsically depend on how many of the unexposed individuals develop exposure after baseline and on the ages at which the exposed individuals developed exposure prior to baseline. These factors pertain to the distribution of risk factor incidence in the population, rather than to the causal effect the risk factor has on disease.