Calculation of fractions of lung cancer incidence attributable to occupational exposure to asbestos and combustion products in Stockholm, Sweden

In a population-based case–referent study of lung cancer we wanted to estimate the over-all influence on the lung cancer incidence from several occupational exposures. Standard methods to do this are based on addition of separately estimated attributable fractions (AFs) by rather complex formulas. Although a simple and valid method for direct estimation of summary effects was published in 1990, it is not well known and has rarely been used. We here describe the method and apply it to the data from the case–referent study. The AF for withdrawal of occupational exposure to both asbestos and combustion products were nearly identical regardless of if it was calculated by an algorithm for summation of AF for the exposure factors separately (6.90%), by a bootstrap method (6.89%, 95% confidence interval, CI: 3.69, 10.04), or by the simple dichotomization-method (6.88%, 95%CI: 3.81, 9.84). The method is very easy to apply to population-based case–referent studies analyzed by logistic regression.