Heritability was estimated for the two phenotypes using the polygenic option in SOLAR (Almasy and Blangero, 1998). The correlation between the total number of DSM-IV criteria endorsed (between 0 and 28) and QUANTDEP was estimated using the Pearson correlation coefficient. Analysis of variance (ANOVA) was used to test if QUANTDEP differed according to the number of substance dependence diagnoses met. We also tested if the average QUANTDEP value differed across alcohol, cannabis, cocaine and opioid dependence diagnoses. Post-hoc pairwise comparisons employed a Tukey correction for multiple testing.
