Dr. Hao plans to conduct an experiment that studies the relationship between smartphone usage and anxiety. He will randomly assign some subjects to the control group in which they will use their phone as usual for a week, and the other subjects to the experimental group in which they will not be allowed to use their phone at all. He will measure the anxiety levels in the two groups at the beginning and end of the study and compare them to see if anxiety levels decreased in the experimental group. In this research, smartphone usage is the .