A researcher was interested in the effect of exercise on memory, but she was concerned males and females would have different reactions to exercise. She first splits the group into males and females, and then randomly assigned half of the male students and half the female students to run up a stairway three times. The other half of the males and females were assigned to rest for an equivalent amount of time. Each student was then asked to memorize a series of random digits. She compared the numbers of digits remembered within the two groups. This study design is best described as: