Professor Boredom has noticed that his students keep falling asleep during his lectures. He thinks that keeping the lights in the lecture hall on at their brightest level will help to keep students awake. He decides to give the same lecture to three groups of people. He holds one class session with bright lights, one class session with normal lighting, and one class session with dim lights. Professor Boredom has a teaching assistant count the number of sleepers at the end of each class. The dependent variable(s) in the above experiment is (are) ......