correlated phenotype (e.g., a component of temperament) could be responsible for the current association findings and those previously reported. While this possibility is difficult to exclude, examinations that have incorporated aspects of temperament have produced mixed results.41,48 Despite the considerably larger size of our sample (more than three-fold larger than most prior association studies of heroin dependence), it is possible that we may have failed to detect significant associations because of limited power (i.e. type II error). Similarly, the smaller size of the neighborhood control subsamples either could be limiting significant differences found in comparisons with cases or between subgroups or could be resulting in spurious associations (i.e., type I error). Finally, the reductions in sample size produced by the more stringent exclusion criteria (i.e., including alcohol or tobacco dependence along with illicit drug dependence) used for the neighborhood control subgroups may have contributed to the weaker observed associations by decreasing power.
