The test group contained 1000 unrelated ND cases and 1000 unrelated comparison subjects, including controls and subjects without ND, but with another substance dependence. Five hundred cases and 500 comparison subjects were EAs, and an additional 500 cases and 500 comparison subjects were AAs. As with the discovery sample, the race of the subjects was confirmed by the analysis of AIMs data. For ND cases, 97.8% were comorbid with cocaine, opioid, or alcohol dependence (49.4% had opioid dependence, 54.3% had alcohol dependence and 85.1% had cocaine dependence). Of the comparison subjects, 27.9% had another substance dependence diagnosis (mostly alcohol dependence). The mean age of the cases (56.7% male) was 40.0 (SD = 9.8) years, and of the comparison subjects (42.1% male) it was 38.8 (SD = 13.1) years.
