A note on evaluating Supplemental Instruction

Selection bias pervades the evaluation of supplemental instruction (SI) in non-experimental settings. Naive impact evaluation typically involves a comparison of observed mean outcomes between those who received the treatment and those who did not. For example, the average or mean final marks of SI participants and nonparticipants may be obtained, and the difference between the two is used as an estimate of the impact of SI. Unfortunately, this approach does not take into account the fact that participation in SI is typically a voluntary decision, and, as such, is influenced by individual characteristics - observed and (crucially) unobserved to the program evaluator - that may also contribute to the final mark. 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. [Author abstract, ed]