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  • Prof. Itai Ashlagi (Stanford University)

    What Matters in School Choice Tie-breaking? How Competition Guides Design (with Afshin Nikzad)
    Date: Oct. 20, 2016.
    Time: 3:30 pm - 5 pm.
    Place: Bunche Hall 9383.

    Abstract: Many school districts apply the student-proposing deferred acceptance algorithm after ties among students are broken exogenously. We compare two common tie-breaking rules: one in which all schools use a single common lottery, and one in which every school uses a separate independent lottery. We identify the balance between supply and demand as the determining factor in this comparison. We first analyze a two-sided matching model with random preferences in over-demanded and under-demanded environments. In a market with a surplus of seats a common lottery is less equitable and there are efficiency trade-offs between the two tie-breaking rules. However, a common lottery is always preferable when there is shortage of seats. The theory suggests that popular schools should use a common lottery to resolve ties. We run numerical experiments with New York City choice data after partitioning the market into popular and non-popular schools. The experiments support our findings.

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