We began psychometric analysis by estimating one and two-factor models for existing DSM-IV criteria with and without craving. We did this to determine unidimensionality of the criteria (i.e., representing a single underlying factor), which is required to accurately estimate the parameters of the most widely used IRT models. EFA of tetrachoric correlations using quartimin rotation (Jennrich and Sampson, 1966) was used, maximizing the sum of row variance of the structure matrix. This rotation was chosen over other possible rotations (e.g., Promax) because it is less likely to produce multiple spurious factors (favoring parsimonious factor structures) while still allowing for correlated factors (Lubke & Muthen, 2007). All latent variable analyses were conducted using MPLUS Version 5.21 (Muthén & Muthén, 2009), which adjusted standard errors for the complex sampling design and uses a weighted least squares mean and variance adjusted (WLSMV) estimator.
