In a final step, we investigated whether the familial confounding was mainly a result of age vs time since first AUD registration. Using the combined data set, we constructed 4 new parameters for years since first AUD registration: 1 to 5, 6 to 10, 11 to 20, and 21 or more years. Using the same modeling framework, we first fitted a stratified Cox regression model allowing these 4 parameters to be independent within each sample and age interval. We then obtained a slope for the familial confounding for each of the 4 parameters, which describe years since registration and within each age interval. All statistical analyses were performed using SAS version 9.3 (SAS Institute Inc).20
