We generated total information curves (TIC) for different criterion sets to show their ability to discriminate individuals along the latent trait severity spectrum (Muthen and Muthen, 1998-2007). Two or more TICs are often visually compared to see whether adding criteria to an initial set adds information. We compared four curves for each substance, representing four sets of criteria: dependence only; dependence and abuse; dependence and craving; and dependence, abuse and craving. A summary of the overall total information provided by each criterion set was indicated by the total information area index (TIA) (De Ayala, 2009), which equals the area under the TIC and is calculated by summing all the respective discrimination parameter estimates (De Ayala, 2009). TIA confidence intervals (CI) were calculated using the delta method (R code available at http://www.columbia.edu/~mmw2177/irtprog.html). Non-overlapping 95% CIs for the TIAs indicate that the addition of more criteria provide significantly more information.
