Evaluating the continuity of symptomatology between depressed and nondepressed individuals

Debate is contentious concerning whether depression should be viewed as a distinct category or as a continuum including overlapping normal and clinical phenomena. A nonparametric item response model was used to evaluate whether the probability of expressing individual symptoms differed between nondepressed and clinically depressed adults experiencing similar levels of overall severity. Even though depressed and nondepressed individuals were equated in terms of overall severity, differences on specific symptoms emerged. Depressed mood, anhedonia, and suicidality were more likely to be expressed in depressed than in nondepressed individuals, whereas hypochondriasis and middle insomnia were more likely to be expressed in nondepressed individuals at similar levels of severity. Such differences are inconsistent with the view of depression as a simple continuum.