Supplemental Instruction (SI) is a voluntary, non-remedial, peer-facilitated, course-specific intervention that has been widely demonstrated to increase student success, yet concerns persist regarding the biasing effects of disproportionate participation by already higher-performing students. With a focus on maintaining access for all students, a large, public university in the Western United States used student demographic, performance, and SI participation data to evaluate the intervention's efficacy while reducing selection bias. This analysis was conducted in the first year of SI implementation within a traditionally high-challenge introductory psychology course. Findings indicate a statistically significant relationship between student participation in SI and increased odds of successful course completion. Furthermore, the application of Coarsened Exact Matching reduced concerns that increased course performance was attributed to an over-representation of higher performing students who elected to attend SI Sessions.