An experiment was performed in a clinic that treated individuals who were having a very hard time sleeping. Each participant in the study was asked to sleep in a room with controlled temperature, bedding, and lighting. Each was given either a sleeping pill, soothing music, or a dog that would lie at the foot of the bed. To determine the treatment each would receive, three different colored chips were placed in a hat; individuals who pulled out a blue chip would get the sleeping pill those who picked a red chip would get the soothing music, and those who picked a green chip would get the dog. Each night, the exact number of minutes of sleep was recorded for each participant. What is the control variable in this experiment ?