Mrs. Smith is teaching Geometry and wants to help her students do their best on the exams. After failing to find any research on different test formats, she decides to conduct her own research. She flipped a coin for each student in her classes. If the coin landed heads up, the student took a multiple-choice test. If the coin landed tails up, the student received a fill-in-the-blank exam. Afterward, Mrs. Johnson compared the averages for the two test formats. What is the variable of interest in this case?