A company is creating a new mosquito repellent, and needs to figure out how much is needed to stop mosquito bites. They find volunteers from the nearby college. Each person is given either 1mL, 2mL, or 3mL of the repellent to rub on their arm. Some volunteers are given no repellent. Each person is then put in a room with mosquitoes for 10 minutes. Numbers of bites are measured for each volunteer.

In Experiment 1 above, what is the independent variable?