A psychologist studies the effects of exam review strategy on long-term memory. College students are randomly selected and then randomly assigned to one of four groups. Group 1 studies using visual information only. Group 2 studies using auditory information only. Group 3 studies using visual and auditory information. Group 4 does not study at all. The students follow the instructions for how they must study for four days to prepare for a 100-question multiple-choice test. They all read the same chapter in the textbook and all take the same exam. Exam scores are then recorded. Identify the different levels of the independent variable.