When less is more: how fewer diagnostic criteria can indicate greater severity.

For diagnosing many mental disorders, the current Diagnostic and Statistical Manual of Mental Disorders (DSM) system weights each diagnostic criterion equally--each criterion counts the same toward meeting the diagnostic threshold. Research on the diagnostic efficiency of criteria, however, reveals that some diagnostic criteria are more useful than others for identifying their associated mental disorders. That some criteria are more useful than others suggests that the criteria may indicate different levels of severity, but this has yet to be empirically tested. Using data from a large epidemiological study (N = 41,227) and two-parameter logistic item response theory models, the level of latent severity associated with each diagnostic criterion for a particular DSM mental disorder was estimated. Maximum likelihood estimates for all possible response patterns to the criteria were then calculated, and results indicated that items and combinations of items identified varying levels of severity. Furthermore, different response patterns associated with the same raw score identified a range (or band) of latent severity. In many instances, these bands overlapped, revealing that some response patterns with fewer endorsed criteria had higher estimated latent severity than did response patterns with more endorsed criteria. Specifically, many response patterns associated with a raw score of 3 (below threshold for the analyzed disorder) indicated greater latent severity than did response patterns associated with a raw score of 4 (at threshold).