Dynamic matching in school choice: efficient seat reassignment after late cancellations (invited talk)

In the school choice market, where scarce public school seats are assigned to students, a key issue is how to reassign seats that are vacated after an initial round of centralized assignment. Every year around 10% of students assigned a seat in the NYC public high school system eventually do not use it, and their vacated seats can be reassigned. Practical solutions to the reassignment problem must be simple to implement, truthful and efficient. I propose and axiomatically justify a class of reassignment mechanisms, the Per- muted Lottery Deferred Acceptance (PLDA) mechanisms, which generalize the commonly used Deferred Acceptance (DA) school choice mechanism to a two-round setting and retain its desirable in- centive and efficiency properties. I also provide guidance to school districts as to how to choose the appropriate mechanism in this class for their setting. Centralized admissions are typically conducted in a single round using Deferred Acceptance, with a lottery used to break ties in each school’s prioritization of students. Our proposed PLDA mechanisms reassign vacated seats using a second round of DA with a lottery based on a suitable permutation of the first-round lottery numbers. I demonstrate that under a natural order condition on aggregate student demand for schools, the second-round tie-breaking lottery can be correlated arbitrarily with that of the first round without affecting allocative welfare. I also show how the identifying char- acteristic of PLDA mechanisms, their permutation, can be chosen to control reallocation. vacated after the initial round are reassigned using decentralized waitlists that create significant student movement after the start of the school year, which is costly for both students and schools. I show that reversing the lottery order between rounds minimizes reassignment among all PLDA mechanisms, allowing us to alleviate costly student movement between schools without affecting the ef- ficiency of the final allocation. In a setting without school priorities, I also characterize PLDA mechanisms as the class of mechanisms that provide students with a guarantee at their first-round assign- ment, respect school priorities, and are strategy-proof, constrained Pareto efficient, and satisfy some mild symmetry properties. Finally, I provide simulations of the performance of different PLDA mecha- nisms in the presence of school priorities. All simulated PLDAs have similar allocative efficiency, while the PLDA based on reversing the tie-breaking lottery between rounds minimizes the number of reassigned students. These results support our theoretical findings. This is based on joint work with Itai Feigenbaum, Yash Kanoria, and Jay Sethuraman.