Whether intended by education policy (e.g., via school entrance examinations) or unintended (e.g., via social stratification), selection effects in education (who goes where, gets what, and how much) shape educational opportunities -- influencing the life chances of individuals and groups, and the structure of societies. However, within the quantitative approach to educational research, current statistical methods can struggle to simultaneously evaluate both the presence and impacts of these effects. In turn, this methodological limitation impedes efforts to facilitate equality of educational opportunity. This paper responds with a critical overview of types of selection effects in education, their consequences for educational opportunity, and the statistical methods used for their identification. Two empirical illustrations show how a new statistical method ("airbag moderation") can enable better detection and evaluation of selection effects in education and help direct future research into selection effects in education with a focus on opportunities for equality of educational opportunity.