(42), learning (43), conceptual judgment (44), personality (45), reaction time (46), manual control (47), and speech (10). Rats, pigeons, goldfish, and monkeys have produced exceptionally neat ROC curves, by rating and yes-no responses, with variations in signal probability or with differential reinforcement (48). Lee B. Lusted has applied the ROC to medical decisions, particularly in radiology (49). An early finding in physiological psychology is that the amplitude of a particular component of evoked cortical potentials increases monotonically with increasing strictness of the decision criterion, whether the criterion is manipulated by varying the signal probability or by varying the values and costs of the possible stimulus-response outcomes. This component was always present when the observer correctly reported a signal to exist, but was never present when the response was a miss or a false alarm (50). The ROC has also been used to evaluate the effectiveness of information retrieval systems (51).
The clinician looking, listening, or feeling for signs of a disease may far prefer a false alarm to a miss, particularly if the disease is serious and contagious. On the other hand, one may believe that the available therapy is marginally effective, expensive, and debilitating. The pilot seeing the landing lights only when they are a few yards away may decide that the plane is adequately aligned with the runway if alone and familiar with that plight. One may be more inclined to circle the field before another try at landing if there are many passengers or recent memory of another plane crashing under those circumstances. The Food and Drug administrator suspecting botulism in a canned food may not want to accept even a remote threat to the public health. But one may be less clearly biased if a recent false alarm has cost a canning company millions of dollars and left some damaged reputations. The making of almost any fine discrimination is beset with such considerations of probability and utility, which are extraneous and potentially confounding when one is attempting to measure the acuity of discrimination per se.
The ROC is an analytical technique, with origins in statistical decision theory and electronic detection theory, that quite effectively isolates the effects of the observer's response bias, or decision criterion, in the study of the discrimination behavior. This capability, pursued through a century of psychological testing, provides a relatively pure measure of the discriminability of different stimuli and of the capacity of organisms to discriminate. The ROC also treats quantitatively the response, or decision, aspects of choice behavior. The decision parameter can then be functionally related to the probabilities of the stimulus alternatives and to the utilities of the various stimulus-response pairs, or to the observer's expectations and motivations. In separating and quantifying discrimination and decision processes, the ROC promises a more reliable and valid solution to some practical problems and enhances our understanding of the perceptual and cognitive phe-