QUANTDEP, the quantitative factor score, was constructed by conducting a confirmatory factor analysis of the 7 DSM-IV lifetime dependence criteria (coded as present or absent) for each of the 4 substances (28 items total). As we were interested in those genetic underpinnings that were common to all dependence criteria across the four substances, we elected to use a single factor confirmatory model and did not conduct exploratory analyses, in addition to limiting the factor analysis to the dependence criteria to exclude abuse. All individuals with DSM-IV criteria data were utilized, regardless of age, or substance use. The factor score from the resulting confirmatory analyses was utilized as the quantitative phenotype.
