Supplemental Instruction (SI) is a national program designed to aid college student learning. Many researchers have noted that analysis of the impact of the SI program on student achievement is problematic as a result of the inherent self-selection bias. We apply a sufficiently sophisticated statistical technique that controls for the self-selection problem and test the effect of student SI attendance in freshmen level courses on graduation success. Our analysis suggests that SI attendance in freshmen level courses has a statistically significant influence on graduation success. Indeed, SI attendance, everything else held constant, increases the probability of timely graduation by approximately 11%.