One key question is whether individual differences in SUDs are quantitative (that is, a matter of degree) or qualitative (different in kind), or arise from a combination of both mechanisms. Statistical methods to distinguish between these processes have advanced considerably in the past decade. Space precludes a detailed review of these statistically sophisticated methods32. Tried and true methods such as factor analysis (exploratory and confirmatory forms) are used to see whether SUD symptoms define clear and sensible factors that can be used as quantitative indices for genetic analysis. Latent class analysis attempts to determine whether drug abuse symptoms covary only because the population consists of two or more heterogeneous groups (that is, affected and unaffected) that differ in symptom rates. Historically, latent factor and class approaches have been used independently and for different purposes, when in reality each represents a competing hypothesis regarding why key SUD symptoms correlate with each other. Factor mixture models combine these latent factor and latent class approaches while circumventing the limitations of each31. In principle, this hybrid modeling strategy may fit diagnostic data better and,
