Because between-subjects experiments use a different group of participants for each treatment condition, one concern is that the participants in one treatment are noticeably different—for example, older, faster, or smarter—from the participants in another treatment. In the Katona experiment, how can the researcher be reasonably confident that the students in the understanding group were not considerably smarter than the students in the memorization group and that the difference in performance isn't simply caused by intelligence? The two groups were roughly equivalent when assigned to the two treatments. The same individuals participated in both treatment conditions.