The most basic single-subject research design. Also called a reversal design, it is ideally suited to show that an organism's behavior is regulated by specific features of the environment. The A-phase, or baseline, is used to measure behavior before the researcher introduces an environmental change. During baseline, the experimenter takes repeated measures of the behavior under study, and this establishes a criterion against which any changes (due to the independent variable) may be assessed. Following the baseline phase, an environmental condition is changed (B phase) and behavior is measured repeatedly. If the independent variable, or environmental condition, has an effect, then the behavioral measure (dependent variable) will change-increase or decrease. Next, the baseline phase is reintroduced (A) and behavior is again measured. Since the treatment is removed, behavior should return to baseline levels. Finally, the independent variable is introduced again and behavior is reassessed (B). According to the logic of the design, behavior should return to a level observed in the initial B-phase of the experiment. This second application of the independent variable helps ensure that the behavioral effect is due to the manipulated condition. Absolute stimulus control. When operant responses are regulated by the physical properties of one stimulus (e.g., a red light), this is called absolute stimulus control. See also relative stimulus control. Activity anorexia. Physical activity decreases food intake and declining food intake increases activity. This feedback loop is called activity anorexia, and a similar cycle occurs in many anorexic patients. Ad libitum weight. The body weight of an organism that has free access to food 24 hours a day. Adjunctive behavior. Also called interim behavior. On interval schedules of reinforcement, or time-based delivery of food, organisms may emit excessive behavior within the interreinforcement interval. For example, rats may drink up to three times their usual daily water intake (polydipsia) over a 1 hour session. This behavior immediately follows reinforcement and is a side-effect of periodic food delivery. Analytical pragmatism. Behavior analysis is based on Skinner's pragmatic or practical philosophy. A behavioral relationship is important if the relation is powerful in its effects and can be used to change the behavior of an individual. Some behavior analysts choose to study behavioral relationships that have direct importance for human affairs. Other researchers investigate contingencies of reinforcement, using operant chambers, with a variety of organisms. Because basic researchers hold a pragmatic view and applied researchers are interested in analysis, both fields share a similar set of principles and philosophical assumptions called analytical pragmatism.