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...