Ex - ante Fairness in the Boston Mechanism under Pre - exam Preference Submission

1 In an exam-based school choice matching mechanism (e.g., China’s college admission), student exam scores are used to determine school priorities. However, a student’s true qualifications or ability may not be perfectly revealed by his/her exam score. The matching outcome can thus be undesirable in the sense that it is not ex-ante fair, i.e., it may not match students with higher intrinsic abilities to better colleges. A Boston mechanism with the requirement that students submit their preferences over colleges before the exam score is realized (i.e., pre-BOS mechanism) can potentially improve ex-ante fairness. We compare the timing of students’ preference submissions being before versus after the exam under either the Boston (BOS) or Serial dictatorship (SD) mechanisms, under the assumption of homogeneous student preferences. We characterize the equilibrium under which the pre-BOS mechanism can achieve ex-ante fairness. We find that a further restriction requiring that students can only include one school in their submission list (i.e., constrained pre-BOS) can implement the ex-ante fair matching outcome more easily than the unconstrained pre-BOS.