The theory of signal detection, originally formulated in the context of vigilance tasks in applied psychology, went on to become useful in a wide variety of other fields, including medicine, psychiatry and engineering. Using the concepts of hit rate, the proportion of signals correctly detected, and false positive rate, the proportion of non-signals reported as signals, two measures d' and ß are derived. The former measures the ease with which the signal can be detected, or the skill shown by the subject in doing so, while ß measures the degree of caution which the subject adopts in reporting signals. Thus, d' is usually a function of the experimental conditions while ß can be manipulated by changing the subject's instructions. Originally published in 1973, this book will be of value to anyone with an interest in signal detection.