This study used a mixed-methods approach to explore the heterogeneity of effectiveness between supplemental educational services (SES) providers in the Chicago Public Schools (CPS). Using surveys of schools and SES providers, and statistical modeling, providers using college graduates and teachers as tutors were found to be more effective than those using college students. Contrary to what research on tutoring suggested, group size was unrelated to student achievement. Furthermore, school ratings of providers were not related to provider effectiveness. The results suggest that part of improving the overall effectiveness of SES may involve increasing the qualifications of tutors. Furthermore, educators should consider the qualifications of tutors when choosing SES providers to work in their schools. Additional formative research on SES should be done to identify additional levers for increasing its impact.