Jeffrey wants to lose weight by using caffeine to help with exercise. He asks himself, “Will caffeine raise the heart rate to help burn more fat?” He tries to test this out by comparing the heart rates of his neighbors, 3 brothers who are triplets. One of the brothers drinks Pepsi, another drinks a double shot of espresso (which is coffee with twice the normal amount of caffeine) and a third brother drinks only water for comparison. Jeffrey finds out that the triplet with the double shot of espresso had the highest heart rate.
55. What is the independent variable for this experiment?