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