Two major sources of heterogeneity in linkage studies arise from differing definitions of smoking behavior and ethnicities of samples, since a variety of smoking behavior assessments were included in the current GSMA across different populations. Although we postulated that some genes that regulate smoking behavior might be independent of specific smoking behavior assessments and population groups, restricting the combined analyses to studies with similar ascertainment criteria or to subjects from similar ethnic backgrounds could potentially increase power to detect linkage to particular regions where the relevant risk loci are specific either to a trait or to a population. Therefore, subgroup analyses of the families assessed by FTND or MaxCigs24 were performed. Subjects of European ancestry were also analyzed separately. Other populations or phenotype groups had too few studies available for separate analysis.
