The focus of this book is on detection and recognition as fundamental tasks that underlie most complex behaviors. As defined here, they serve to distinguish between two alternative, confusable stimulus categories, which may be perceptual or cognitive categories in the psychology laboratory, or different states of the world in practical diagnostic tasks.
The task of detection is to determine whether a stimulus of a specified category (call it category A) is present or not; a "signal" is to be distinguished from a stimulus from category B, consisting of background interference, or "noise." The task of recognition is to determine whether a stimulus known to be present, as a signal, is a sample from signal category A or a signal category B. The task of diagnosis can be either a detection or recognition task, or both. For example, Is there something abnormal on this X-ray image, and, if so, does it represent a malignant or benign condition? In the laboratory, the objective is to measure the acuity with which organisms can make perceptual or cognitive distinctions. In diagnostics, the objective is to assess the acuity or accuracy of human observers and of devices, working by themselves or in combination.
As a result of applying experimentally the concepts of signal detection theory, present understanding of these tasks is that they involve two independent cognitive processes--one of discrimination and one of decision. A discrimination process assesses the degree to which the evidence in an observation favors the existence of a signal (versus noise alone) or of Signal A (versus signal B). A decision process determines how strong the evidence must be in favor of alternative A to make response A, and makes a choice of A (or B) after each observation depending on whether the requisite evidence is met. Strength of evidence lies along a probabilistic continuum, and the decision maker sets a cutoff along the