A study from the Student Achievement Foundation reported that students who participate in after-school tutoring programs score higher on standardized tests compared to those who do not participate. The foundation claims that this indicates a direct benefit of after-school tutoring on academic performance.
However, some educators argue that the students who choose to attend after-school tutoring programs may already be more academically inclined than their peers. This suggests that the observed increase in test scores might not solely be due to the tutoring but rather the pre-existing capabilities of the students themselves.
Which of the following statements would most strengthen the argument that after-school tutoring programs directly lead to higher standardized test scores?