After confirming unidimensionality, we fit the 2-parameter logistic item response theory (IRT) model to all of the abuse and dependence criteria as well as craving (seeEmbretson and Reise (2000)) for an approachable textbook of IRT methods; the introduction of Langenbucher et al. (2004) for abbreviated summary of these methods, and Shmulewitz et al. (2011) for a glossary of IRT terminology). Because of the low prevalence and likely deletion of legal problems, we also fit the IRT model without legal problems. Criteria were also ranked by their estimated severity. We generated item characteristic curves (ICC) to display the estimated probability of each criterion across the underlying continuum. In the ICC, the severity parameter is the point on the x-axis where the probability of endorsing a criterion is 0.5, and discrimination is the slope of the curve at that point. Steeper slopes indicate greater discrimination. All IRT estimates were obtained using marginal maximum likelihood in the latent trait modeling (“ltm”) package in R2.10.1 (Rizopoulos, 2006).
