For the linkage analysis of the 14 SSAGA nicotine dependence symptoms, the support intervals for reported linkage peaks were defined as the region surrounding a linkage peak yielding a LOD score that was greater than the maximum LOD – 1 in each direction. Only these support intervals were subjected to linkage analysis to reduce the number of statistical tests. Further, LOD scores were only interpreted as a measure of magnitude and were not used to determine significance given the exploratory and descriptive nature of these analyses. Because the current sample was selected for alcohol dependence, which is highly correlated with tobacco use (Miller and Gold, 1998), prevalence rates for nicotine dependence and its respective symptoms were estimated in the unselected control sample and included in the tested models to correct for ascertainment bias when calculating h2 and linkage. Finally, allele frequencies used to calculate allele sharing probabilities between relative pairs were estimated using study participants from varying ethnic backgrounds. This approach can lead to biased allele frequency and sharing probabilities amongst minority participants. Thus, analyses were conducted for the full
