Carl wants to know which sports drink is most effective at replenishing electrolytes during intense physical activity. He decides to test 4 different brands of sports drinks: Gatorade, Vitamin Water, PowerAde, and Propel Water. He tests the different sports drinks on the varsity boys swim team at his school. In his experimental group there are 5 different 17-year-old boys. They are all practicing for the 100-meter freestyle race. Each boy tests each of the different brands of sports drinks, and Carl measures their electrolyte concentration in their blood.