Background: Rigorous meta-analysis holds promise to answer the ultimate important question in education: which interventions work and for whom? Evidence-based reform in education has contributed to a proliferation of well-designed experiments in recent years, which calls for updated systematic reviews. However, this proliferation is a double-edged…

Selection bias pervades the evaluation of supplemental instruction (SI) in non-experimental settings. This brief note provides a formal framework to understand this issue. The objective is to contribute to the accumulation of credible evidence on the impact of SI.