Male controls for the follow-up study were drawn from the KORA study (N=382) and from a population based sample (N=614) which had been collected in the area of Bonn by one of the authors (MR) for use as a control sample for association studies within the framework of the NGFN. Controls were not stratified for alcohol abuse/dependence, which may introduce a conservative bias. Tests for genetic differentiation between German populations have shown low levels of population substructure, thus demonstrating that the German population is an appropriate source for use in association studies of complex diseases.40
