Using item response theory to understand comorbidity among anxiety and unipolar mood disorders

The authors hypothesized that anxiety and unipolar mood disorders are often comorbid because each disorder indicates a broad, higher order factor. In a clinical subsample of the nationally representative National Comorbidity Survey participants (N = 251), a one-factor model fit the correlations amon g 7 dichotomous anxiety and unipolar mood diagnoses. Following the lead pro vided by literature on the structure of emotional and behavioral problems i n children, we labeled this factor internalizing. Item response theory was used to explore how each diagnosis mapped onto the internalizing factor. Th e test information function derived from the 7 diagnoses suggested that the y measure primarily the higher end of the factor. In addition, very high sc ores on internalizing (meeting criteria for 6-7 disorders) were associated with increased social costs, a phenomenon not well captured by the "comorbi dity" concept. The results underscore the need to develop clinical assessme nt instruments that span the full range of the internalizing factor and mea sure both the shared and distinctive features of anxiety and unipolar mood disorders in a graded, continuous fashion.