State the three variables that were controlled in the study?

Dr. Aralem teaches an introductory psychology course at the local college. He is convinced that his students are not getting enough sleep, leading to poorer grades in their college courses. Dr. Aralem starts each Monday and Wednesday lecture with a short quiz. He finds that students usually score higher on the Wednesday quiz than on the Monday quiz. Dr. Aralem hypothesizes that the poor performance on the Monday quiz may be due to insufficient sleep after late weekend nights. To test his idea, Dr. Aralem conducts a study. During the Monday cl, he asks students willing to participate in the study to raise their hands. To guard against bias, students are not told until the end of the study about its purpose. Sixty students volunteer and are divided into four groups of equal size based on a survey of their sleep habits. In one group, students had no more than four hours of sleep the night before last Monday’s quiz. Students in a second group had at least seven hours of sleep before last Monday’s quiz. Students in a third group had four or fewer hours of sleep before last Wednesday’s quiz; the fourth had at least seven hours of sleep before last Wednesday’s quiz. Dr. Aralem found that students in the two groups with four or fewer hours of sleep performed significantly worse on the next day’s quiz than students in the two groups who had seven or more hours of sleep. Dr. Aralem concluded that his hypothesis was correct: Insufficient sleep leads to poor college grades.