Questionnaires generate numerical scores, but endorsing different sets of items could produce the same score despite reflecting qualitatively different configurations of clinical features. Formal psychological assessment (FPA) attempts to overcome this by identifying the clinical features entailed by observed response patterns. This study illustrates an application of FPA to the cleaning subscale of a questionnaire assessing obsessive-compulsive symptoms and DSM-IV-TR diagnostic criteria for obsessive-compulsive disorder. A deterministic model of the items-criteria relationships was constructed by mapping each item to each diagnostic criterion. The resulting model was tested on a large community sample (N = 4,412). Results indicate that the theoretical model has adequate fit; item error rates and probabilities for each of the criteria are examined. Clinically relevant examples of the items-criteria relationships are discussed. Possible applications of FPA to personality assessment are also discussed, including long multidimensional questionnaires and questionnaires that use subtle item content.