"Discriminative capacity" is defined as a property of a test, measuring device or scholastic exam, which enables us to segregate and categorize objects or people according to their measured values. The concept, partially derived from Ferguson’s index of classificatory power, is developed upon three bases : the probability of categorizing an object (or person) in its proper measuring interval; the sufficient length of measuring intervals; the number of efficacious intervals in an empirical or theoretical distribution of measures. Expressed as a function of the reliability coefficient of a measuring device, discriminative capacit appears as a brand new tool in the conceptual apparatus of classical test theory.