Zion is investigating the effectiveness of his summer math tutoring program. He randomly assigns elementary school students to a tutoring group, which receives the math tutoring program 3 days a week during the entire break, or a self-study group, which receives a sample of math materials without a tutor. At the end of the summer, all of the students take the same math test with a possible score of 0 to 100. The students in the tutoring group are proctored by a strict and intimidating teacher, while the students in the self-study group are proctored by a friendly and approachable teacher. Zion finds these results: d = 0.15, 95% CI [.30, .45]. What is the most likely explanation for Zion's results? insensitive measures design confounds acting in reverse floor effects weak manipulations