Two-parameter IRT analysis then was conducted to determine the unidimensionality (one factor) of each dependence, as well as item-level discrimination and severity (Edwards and Gross, 1976; Wu et al., 2009a). Seven substance-specific dependence criteria (dichotomous variables) among past-year users of the corresponding substance were analyzed; hallucinogen use (6.3% of the sample, n=58) and inhalant use (1.2%, n=11) were not examined. The two-parameter IRT model assumes that item responses are a function of discrimination and severity of the items included. Item severity (threshold) is defined by the location on a latent continuum at which an item has a 50% likelihood of being present; item discrimination measures the precision with which an item differentiates between individuals with levels of the latent trait above or below its severity level.
