Single equation regression models have been used to test the effect of Supplemental Instruction (SI) on student retention. These models, however, fail to account for the two salient features of SI attendance and retention: (1) both SI attendance and retention are categorical variables, and (2) are jointly determined endogenous variables. Adopting primarily from the economics education literature, this article applies the bivariate probit model, a model appropriate for the case of simultaneous binary choice, to analyze the effect of SI attendance on retention. It is demonstrated that single equation methods are likely to overestimate this effect. Definition of Variables are appended. (Contains 2 tables and 1 footnote.)