15. Jessie is concerned about how tired his classmates seem. He created
a drink that he thinks will boost their energy. He recruits 100 classmates
with a history of tiredness. He has 50 of them drink his new special energy
drink. The other 50 drink a beverage that looks like the special energy
drink, but is just water and food coloring. Both groups were told that they
were getting the special drink that would boost energy. After 2 hours, 30
classmates in the first group reported having more energy and 8
classmates in the second group reported having more energy. What is the
independent variable?