under conditions of uncertainty. In foggy circumstances, we are forced to decide how far away from us an object is, based solely upon visual stimulus which is impaired by the fog. Since the brightness of the object, such as a traffic light, is used by the brain to discriminate the distance of an object, and the fog reduces the brightness of objects, we perceive the object to be much farther away than it actually is (see also decision theory). According to SDT, during eyewitness identifications, witnesses base their decision as to whether a suspect is the culprit or not based on their perceived level of familiarity with the suspect. To apply signal detection theory to a data set where stimuli were either present or absent, and the observer categorized each trial as having the stimulus present or absent, the trials are sorted into one of four categories: {| class="wikitable" |- ! ! Respond "Absent" ! Respond "Present" |- ! Stimulus Present | Miss | Hit |- ! Stimulus Absent | Correct Rejection | False Alarm |} Based on the proportions of these types of trials, numerical estimates of sensitivity can be obtained with statistics like the [[sensitivity index|sensitivity index d]] and A', and response bias can be estimated with statistics like c and β. Signal detection theory can also be applied to memory experiments, where items are presented on a study list for later testing. A test list is created by combining these 'old' items with novel, 'new' items that did not appear on the study list. On each test trial the subject will respond 'yes, this was on the study list' or 'no, this was not on the study list'. Items presented on the study list are called Targets, and new items are called Distractors. Saying 'Yes' to a target constitutes a Hit, while saying 'Yes' to a distractor constitutes a False Alarm. {| class="wikitable" |- ! ! Respond "No" ! Respond "Yes" |- ! Target | Miss | Hit |- ! Distractor | Correct Rejection | False Alarm |} Applications Signal Detection Theory has wide application, both in humans and animals. Topics include memory, stimulus characteristics of schedules of reinforcement, etc. Sensitivity or discriminability Conceptually, sensitivity refers to how hard or easy it is to detect that a target stimulus is present from background events. For example, in a recognition memory paradigm, having longer to study to-be-remembered words