Non-compliance, Bayesian Inference and School Vouchers: A Thesis Defense 

 Proceedings in the Harvard Dept. of Statistics seminar series started early this year, as Hui Jin eloquently delivered her doctoral thesis defense on Wednesday, September 14, entitled "Principal Stratification for Causal Inference with Extended Partial Compliance."  Jin applied her ideas both to drug trials and to school choice (voucher) programs.  She spoke in particular about the second application, focusing on a study of vouchers as offered to students from low-income families in the New York City public school system.  In this study, 1000 students were offered a subsidy to help pay tuition for a private school of their choice, and were matched with students with similar conditions who were not offered the grant. Both groups were tracked for three years, and a set of tests at the beginning and end were used to measure achievement. The compliance factor was whether grant recipients would always take advantage of the offer, and whether unlucky ones would never make their own way to private school. While the compliance rate after three years remained high - roughly 80% - it was the compliance factor that proved to be the most instructive on the achievement pattern of students, a result found by stratifying the outcomes according to compliance patterns. 

 Those students expected to comply perfectly - attend private school with the grant and public school without it, in all three years - made the  least  improvement as compared to their colleagues in the other strata. Comparative performance improved with non-compliance; the biggest non-conformers, those who attended private or public school regardless of whether the grant was offered showed the most improvement over their previous scores. 

 Notably, the reasons for this performance haven't been completely explained, though Prof. Rubin (Jin's advisor and collaborator on the project) suggests that perhaps using the voucher as a threat to remove a student from his friends may compel a higher performance at public school.  Whatever the underlying mechanism, the results give strong and compelling reason to fully consider the effect of vouchers in the school system.