This article describes the construction of a new self-report scale based directly on the DSM-III criteria for major depressive disorder. It provides data on the interconvertability of scores on this scale with scores obtained on the Beck Depression Inventory, the Zung Depression Scale, the Center for Epidemiologic Studies-Depression (CES-D) Questionnaire, and the Minnesota Multiphasic Personality Inventory Depression Scale (MMPI-D). The article also examines the relation between physical principles of measurement as described by physicists and psychiatric measurement. It concludes with an analysis of the connection between the dichotomies typical of diagnostic statements and the continuous, probabilistic data typical of self-report instruments.