Effects of single-sex schooling in the final years of high school : a comparison of Analysis of Covariance and Propensity Score Matching

Typically, the effects of single-sex schooling are small at best, and tend to be statistically non-significant once pre-existing differences are taken into account. However, researchers often have had to rely on observational studies based on small non-representative samples and have not used more advanced propensity score methods to control the potentially confounding effects of covariates. Here, we apply optimal full matching to the large historical longitudinal dataset best suited to evaluating this issue in US high schools: the nationally representative High School and Beyond study. We compare the effects of single-sex education in the final 2 years of high school on Grade 12 and post-secondary outcomes using the subsample of students attending Catholic schools (N = 2379 students, 29 girls' schools, 22 boys' schools, 33 coeducational schools) focusing on achievement-related, motivational and social outcomes. We contrast conventional Analysis of Covariance (ANCOVA) with optimal full matching based on the propensity score that provides a principled way of controlling for selection bias. Results from the two approaches converged: When background and Year 10 covariates were controlled, uncorrected apparent differences between the school types disappeared and the pattern of effects was very similar across the two methods. Overall, there was little evidence for positive effects of single-sex schooling for a broad set of outcomes in the final 2 years of high school and 2 years after graduation. We conclude with a discussion of the advantages of propensity score methods compared to ANCOVA.