IRT severity and discrimination parameter estimates for each item were used to calculate AUD severity scores for each individual based upon their pattern of responses. Because the factor analysis showed that the DSM-IV abuse and dependence symptoms form a single factor, the abuse and dependence symptoms were used conjointly. IRT-based scores which consider the endorsed symptoms’ severity signals are more precise than, but strongly correlated with, total symptom count. The increased precision results from considering the severity signal (see Figure 2) of the symptoms that were endorsed, in addition to the total number of symptoms. Table 2 shows average severity of AUDs displayed according to the DSM-IV diagnostic categories (i.e., no diagnosis, diagnostic orphans, abuse diagnosis, dependence diagnosis (with 12-month clustering)). IRT-based severity estimates indicate that those without a diagnosis are the least severe (mean = −0.22), diagnostic orphans are more severe (mean = 0.77), followed by abuse diagnoses (mean = 1.44) and dependence diagnoses (mean = 2.60). A test for mean severity differences across categories was highly significant (total symptom count: F=1729.22, p<.001, η2=.70; IRT-based severity estimates: F=1694.42, p<.001, η2=.69). These results suggest that, on average, the current diagnostic algorithm discriminates severity of AUDs.
