Are All Gains from Supplemental Instruction Due to Selection Bias?

Supplemental Instruction (SI), an academic support program that targets difficult introductory college level courses, is praised for increasing grades by half a letter grade. We rigorously evaluate SI in introductory economics and biology courses at a regional university in the mid-Atlantic. Students who attended SI review sessions were up to 65 percent less likely to earn a D, F, or W, and they earned higher grades by a full letter grade. However, participation in SI was low; only 14 percent of students attended multiple SI review sessions, and higher achieving students were more likely to attend sessions. Any increases in grades are concentrated among biology courses and high-achieving students. In conclusion, we find that grade improvements from SI are due to positive selection bias, with limited evidence of causality. While SI might be beneficial for some students, it is less clear whether this program is the most cost-effective approach for struggling students.