A researcher was interested in the impact of exercise on tumor growth in mice. She took 50 mice with tumors and measured the size of their tumors. She then had them walk each day on the treadmill for 6 weeks. All of the mice were fed the same diet, held under the same environmental conditions and required to walk the same amount on the treadmill. At the end of the experiment, she measured the size of their tumors and calculated the change in size. What is the independent variable in this experiment?