Three boys with autism, ages 7–10 years old, served as participants in a study. During baseline, staff used response blocking when five instances of aggression or head-banging occurred within 10 seconds, until attempts ceased for 1 minute. During baseline and treatment, each occurrence of aggression and head-banging was recorded daily and converted to the number of responses per hour. Treatment comprised a differential reinforcement of incompatible behavior (DRI) schedule coupled with response blocking after every head-banging or aggression. The staff initially applied the treatment to head-banging, while continuing to take a baseline on aggression. After noting treatment effects on head-banging, the staff then applied the treatment to aggression.

What is the independent variable?