In terms of discrimination, uncontrolled substances consistently ranked as the least discriminating item regardless of age or sex. This is demonstrated by the shallow slopes of its IRFs and nearly flat IIFs in Figure 1. Thus, the uncontrolled substances item provides less information, while the other substances (those items with greater discriminations, steeper IRF slopes, and higher IIF peaks) provide more information over much narrower trait level ranges. For example, marijuana and tobacco, which rank as the most discriminating items in Table 3 at ages 14 and 17 (with the possible exception of the alcohol item for 17-year-old males), clearly have IIFs with the highest peaks and narrowest ranges at those ages in Figure 1.
