Researchers in Canada performed an experiment with university students to examine the effects of multitasking on student learning. The 40 participants in the study were asked to attend a lecture and take notes with their laptops. Half of the participants were randomly assigned to complete other online tasks not related to the lecture during that time. These tasks were meant to imitate typical student Web browsing during classes. The remaining students simply took notes with their laptops. At the end of the lecture, all participants took a comprehension test to measure how much they learned from it. The results: students who were assigned to multitask did significantly worse (11%) than students who were not assigned to multitask. Identify one variable that the researchers kept the same for all subjects. Provide two reasons why it was important for the researchers to keep this variable the same.